High power short pulse fiber laser

ABSTRACT

A pulsed laser comprises an oscillator and amplifier. An attenuator and/or pre-compressor may be disposed between the oscillator and amplifier to improve performance and possibly the quality of pulses output from the laser. Such pre-compression may be implemented with spectral filters and/or dispersive elements between the oscillator and amplifier. The pulsed laser may have a modular design comprising modular devices that may have Telcordia-graded quality and reliability. Fiber pigtails extending from the device modules can be spliced together to form laser system. In one embodiment, a laser system operating at approximately 1050 nm comprises an oscillator having a spectral bandwidth of approximately 19 nm. This oscillator signal can be manipulated to generate a pulse having a width below approximately 90 fs. A modelocked linear fiber laser cavity with enhanced pulse-width control includes concatenated sections of both polarization-maintaining and non-polarization-maintaining fibers. Apodized fiber Bragg gratings and integrated fiber polarizers are included in the cavity to assist in linearly polarizing the output of the cavity. Very short pulses with a large optical bandwidth are obtained by matching the dispersion value of the fiber Bragg grating to the inverse of the dispersion of the intra-cavity fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/814,628, filed Jun. 14, 2010, titled “HIGH POWER SHORT PULSE FIBERLASER,” which is a continuation of U.S. patent application Ser. No.10/814,319, filed Mar. 31, 2004, titled “HIGH POWER SHORT PULSE FIBERLASER,” now U.S. Pat. No. 7,804,864; all of which are herebyincorporated by reference herein in their entirety.

BACKGROUND

1. Field

The present teachings relate to waveguide-based lasers, such as fiberlasers, that output high power short laser pulses. More particularly,the present teachings relate to pulsed lasers that provide improvedperformance such as reduced pulse width and that preferably includemodular designs that are compact and rugged.

This invention relates to modelocked fiber lasers and more particularlyan ultra-compact integrated fiber laser with pulse width control inconjunction with concatenated sections of polarization maintaining andnon-polarization maintaining fiber sections.

2. Description of the Related Art

High power laser sources are of interest for practical applications invarious fields. High peak power pulsed lasers are desirable, forinstance, in medical and industrial applications, remote sensingapplications, and in optical parametric oscillators. Some specificexemplary applications include use as pump sources for opticalamplifiers and Raman lasers for use in medicine and spectroscopy.Rare-earth-doped double clad fiber lasers offer an excellent combinationof high power and special beam quality that may be particularly useful.

Various of the following references discuss laser systems and are herebyincorporated herein by reference:

-   [1] U.S. patent application Ser. No. 09/576,772 filed on May 23,    2000 by M. E. Fermann, A. Galvanauskas, and D. Harter entitled    “Modular, high energy, widely-tunable ultrafast fiber source”    (Docket No. IM-83);-   [2] U.S. patent application Ser. No. 10/627,069 filed on Jul. 25,    2003 by M. E. Fermann, and G. C. Cho entitled “Polarization    Maintaining Dispersion Controlled Fiber Laser Source Of Ultrashort    Pulses” (Docket No. IMRAA.021A/IM-99);-   [3] U.S. Pat. No. 6,151,338 issued to S. G. Grubb, D. F. Welch,    and R. Zanoni in Nov. 21, 2000 entitled “High power laser optical    amplifier system”;-   [4] O. G. Okhotnikov, L. A. Gomes, N. Xiang. T. Jouhti, A. K.    Chin, R. Singh, and A. B. Grudinin, “980-nm picosecond fiber laser”    IEEE Photonics Technology Letters, 15 (11), 1519-1521 (2003); and-   [5] H. Lim, F. O. Ilday, and F. W. Wise, “Generation of 2-nJ pulses    from a femtosecond ytterbium fiber laser” Optics Letters, 28 (8),    660-662 (2003).

Currently, conventional pulse lasers have practical limitations. Forexample, the optical components in conventional commercial short andultra-short pulse lasers are normally mounted mechanically on a mountingplate such as a breadboard. Such mounting of components can result in abulky packaging and can suffer from mechanical vibrations andenvironmental instabilities. Additionally, such mounting requiresfrequent alignment to achieve the optimum performance. Thus there is aneed for improved lasers and laser systems.

Modelocked fiber lasers are increasingly displacing traditionalsolid-state lasers in ultrafast optic applications. Modelocked fiberlasers can be packaged in very small spaces and also exhibit superiormechanical and thermal stability. In particular, passively modelockedfiber lasers allow compact designs because of the absence of bulkyoptical modulators. Fiber laser systems are pumped using diode laserswith an emission wavelength shorter than the fiber laser emissionwavelength. For upconversion-type fiber lasers, the pump wavelength canbe longer than the emission wavelength. Generally, similar to themajority of all laser systems, the pump wavelength differs from theemission wavelength; a fact which is well known in the art.

Passively modelocked fiber lasers often comprise saturable absorbers toinitiate and stabilize the pulse formation process. Examples of lasersystems using saturable absorbers in this manner are described in U.S.Pat. No. 5,689,519 ('519) to Fermann et al., and U.S. Pat. No. 5,448,579('579) to Chang et al.

Semiconductor saturable absorbers have been implemented in modelockedlasers for a long time. Of particular interest are multiple-layerheterostructures as suggested in U.S. Pat. No. 4,860,296 ('296) to D. S.Chemla et al. However, these early saturable absorber designs wererestricted in that they contained nonlinear layers with a spacing ofexactly an integer multiple of a predetermined optical period. Moreover,the incorporation of multiple layer heterostructures as suggested by'296 relied on semiconductor layers with a thickness of less than 500 Åin order to exploit quantum-confinement effects. Such thin semiconductorlayers generally restrict the bandwidth over which pulse shaping ispossible with saturable absorbers.

A more workable saturable absorber solution was suggested in U.S. Pat.No. 6,252,892 ('892) to Jiang et al., where a resonant saturableabsorber for passive modelocking of lasers was described. Moreover, '892suggests distributed resonant saturable absorbers comprising layers ofsaturable absorber material separated by semiconductor layers notrestricted to a thickness of less than 500 Å. Semiconductor layers witha thickness greater than 500 Å are indeed useful for maximizing thepulse shaping action of saturable absorbers.

As is well known in the art of passive modelocking of color centerlasers (Islam et al., IEEE J. Quantum Electron. Vol. 25, pp. 4254(1989)), the optically excited carriers in semiconductor saturableabsorbers generally relax with different time constants. A first timeconstant of approximately 300 fs depends on the charge carrier densityand excess energy of the hot photo-excited carriers due to intrabanddynamics, e.g. thermalization and cooling of hot carriers to the bandedge. A second longer time constant of 1 ps-30 ns is due to interbanddynamics, e.g. the recombination of the carriers.

These different time constants can be easily realized if the hot chargecarriers are excited well above (about an optical phonon energy above)the band edge. However, when the carriers are photo-excited at the bandedge, the intraband contribution becomes weak due to the low carriertemperature. The excitation near-band edge is usually preferred insaturable absorber design because of the resulting resonant enhancementof the optical nonlinearity. In this case, the nonlinear opticalresponse is governed by the interband dynamics including trap centerassisted recombination and carrier relaxation with two different timeconstants cannot necessarily be observed and moreover, the ratio ofcarrier centers relaxing at the two different time constants cannot becontrolled.

The interband dynamics are generally manipulated by introducing trapcenters for photo-excited charge carriers either by arsenic anti-sitesin GaAs-related material systems grown at low temperature or byimplantation with ions. It has been readily reported (A. R. Hopfel, Ch.Teissl, and K. F. Lambrecht, Appl. Phys. Lett. 53, p. 12581 (1996)) thatthe trapping rate dominate the intraband dynamics in InP implanted with200 keV protons (H⁺) at a dose of 1×10¹⁶ cm⁻², when excited with 1.7 eVphotons. The carrier trap time can be sub 100 fs and the cw luminescenceshows a non-Fermi distribution, indicating the hot carriers undergo arecombination process before they cool down to the band edge.

For ultrafast fiber lasers modelocked by saturable absorbers asdescribed in U.S. Pat. No. 6,252,892 it was shown that cw modelocking isinitiated by Q-switched mode-locking in the very early stages of pulseformation. Hence, Q-switch pulses in the cavity are used for the startof modelocking and the support of Q-switch pulses by a slow opticalmodulation process in the absorber is useful.

Hence, the first longer time constant can be used to initiate pulseformation, whereas the second shorter time constant can be used tostabilize the oscillation of short femtosecond pulses. However, to dateno control of the ratio of carriers relaxing at these time constants waspossible.

In fiber lasers, soliton shaping and or nonlinear polarization evolutioncan further be used to stabilize pulse formation as described in '519.However, to compete on an equal level with modelocked solid state lasersin ultrafast optics applications, modelocked fiber lasers should includethe following: 1) the output polarization state should preferably bewell defined, 2) the construction of the fiber laser should preferablybe adaptable to mass production, 3) the required optical elements shouldpreferably be as inexpensive as possible, and 4) the design conceptshould preferably comprise saturable absorbers with well controllableparameters. It is with respect to these four factors that current,conventional, modelocked fiber laser technology still needs improvement.

Early modelocked fiber laser designs, as exemplified in '519, relied onnon-fiber components for stable operation. Although these earlymodelocked fiber lasers could further accommodate devices that enabledwavelength tuning, a fiber pig-tailed output signal with a well-definedpolarization state was not easily attainable. Similarly, '579 alsoincluded bulk optical components.

Improvements in the basic design of modelocked fiber lasers were madepossible by the use of fiber Bragg gratings to control the dispersioninside the cavity or as replacements for cavity-end mirrors inFabry-Perot-type cavity designs (U.S. Pat. No. 5,450,427 ('427) toFermann et al.). Moreover, the incorporation of polarization maintainingfiber was further suggested in '427 to limit the sensitivity of thecavity to mechanical perturbations of the fiber. These designs allowedcompact wavelength-tunable set-ups as well as synchronization toexternal electronic clocks. Wavelength tunable passively modelockedfiber lasers were later also described in U.S. Pat. No. 6,097,741 ('741)and No. 6,373,867 ('867) to Lin et al.

Further improvements became possible by constructing cladding-pumpedmodelocked fiber lasers (U.S. Pat. No. 5,627,848 ('848) to Fermann etal.).

The need for bulk polarizers was eliminated by the implementation ofall-fiber polarizers as disclosed in U.S. Pat. No. 6,072,811 ('811) toFermann et al. Such integrated modelocked fiber lasers could alsoincorporate fiber Bragg gratings for output coupling. The use of fiberBragg gratings and all-fiber polarizers in the absence of any non-fiberpolarization manipulating elements constituted a great simplificationcompared to single-polarization fiber lasers as discussed by DeSouza etal. (Electron. Lett., vol. 19, p. 679, 1993).

Limitations in integrated cavity designs arose from the need for fiberBragg gratings written in polarization maintaining fiber to produce alinear polarization state of the output pulses. A high degree of laserintegration has also been accomplished in the subsequent '741 and '867patents. These designs lack high polarization extinction, all-fiberelements for polarization selection, and they rely on severalconcatenated intra-cavity polarization-maintaining fiber elements ofextended length, which can induce the generation of satellite pulses atthe fiber output. Indeed, as described in U.S. patent application Ser.No. 09/809,248, in the presence of concatenated fiber sections, pulsestability requires the single-pass group delay between the polarizationaxes of each fiber section to be larger than the generated pulse width.This is required to prevent any coherent interaction of intra-cavitypulses propagating along the two polarization axes at any couplingpoint, e.g., fiber splices. Such coherent interactions can generallyproduce temperature and fiber stress dependent instabilities, which arepreferably avoided. Similarly, no all-fiber elements for controlling thespot size on an intra-cavity saturable absorber were described in '741and '867.

Another method for producing an integrated cavity was introduced bySharp et al. (U.S. Pat. No. 5,666,373 ('373)) where the use of asaturable absorber as an output coupler is described. A limitation withsuch designs is the required precision-polishing and AR-coating at theback-end of the saturable absorber to avoid the formation of satellitepulses inside the cavity.

The construction of high-power modelocked fiber lasers, as enabled bythe use of multi-mode fibers inside a fiber laser cavity, is taught inU.S. Pat. No. 6,275,512 ('512) to Fermann et al.

A passively modelocked fiber laser particularly suitable for producingpulses with a bandwidth approaching the bandwidth of the gain medium wassuggested in U.S. Pat. No. 5,617,434 ('434) to Tamura et al. where fibersegments with opposing dispersion values were implemented. This designhas limited functionality due to the presence of at least two longlengths of fiber with different dispersion coefficients for dispersioncompensation, as well as the presence of non-polarization maintainingfiber, greatly complicating polarization control inside the cavity.

The design principles used in the patents mentioned above werereiterated in a series of recent patents and applications to Lin et al.(U.S. Pat. No. 6,097,741; U.S. Pat. No. 6,373,867, and Application No.US2002/0071454). The designs described in U.S. Pat. Nos. '741 and '867lack appropriate all-fiber, high polarization extinction, polarizingelements that are generally required to minimize the formation ofsatellite pulses at the fiber output. Moreover, these patents do notdescribe all-fiber means to control the spot size on the intra-cavitysaturable absorber; control of the spot size is required to optimize thelife-time of the saturable absorber. Equally none of the prior artdescribes ion-implanted saturable absorber designs with controlled iondepth penetration.

SUMMARY

One embodiment of the invention comprises a pulsed fiber laseroutputting pulses having a duration and corresponding pulse width. Thepulsed laser comprises a modelocked fiber oscillator, an amplifier, avariable attenuator, and a compressor. The a modelocked fiber oscillatoroutputs optical pulses. The amplifier is optically connected to themodelocked fiber oscillator to receive the optical pulses. The amplifiercomprises a gain medium that imparts gain to the optical pulse. The avariable attenuator is disposed between the modelocked fiber oscillatorand the amplifier. The variable attenuator has an adjustabletransmission such that the optical energy that is coupled from themode-locked fiber oscillator to the amplifier can be reduced. Thecompressor compresses the pulse thereby reduces the width of the pulse.Preferably a minimum pulse width is obtained.

Another embodiment of the invention comprises a method of producingcompressed high power short laser pulses having an optical power of atleast about 200 mW and a pulse duration of about 200 femtoseconds orless. In this method, longitudinal modes of a laser cavity aresubstantially mode-locked to repetitively produce a laser pulse. Thelaser pulse is amplified. The laser pulse is also chirped therebychanging the optical frequency of the optical pulse over time. The laserpulse is also compressed by propagating different optical frequencycomponents of the laser pulse differently to produce compressed laserpulses having a shortened temporal duration. In addition, the laserpulse is selectively attenuated prior to the amplifying of the laserpulse to further shorten the duration of the compressed laser pulses.

Another embodiment of the invention comprises a method of manufacturinga high power short pulse fiber laser. This method comprises mode-lockinga fiber-based oscillator that outputs optical pulses. This methodfurther comprises optically coupling an amplifier to the fiber-basedoscillator through a variable attenuator so as to feed the opticalpulses from the fiber-based oscillator through the variable attenuatorand to the amplifier. The variable attenuator is adjusted based on ameasurement of the optical pulses to reduce the intensity of the opticalpulses delivered to the amplifier and to shorten the pulse.

Another embodiment of the invention comprises a pulsed fiber laseroutputting pulses having a pulse width. The pulsed fiber laser comprisesa modelocked fiber oscillator, an amplifier, and a spectral filter. Themodelocked fiber oscillator produces an optical output comprising aplurality of optical pulses having a pulse width and a spectral powerdistribution having a bandwidth. The amplifier is optically connected tothe modelocked fiber amplifier for amplifying the optical pulses. Thespectral filter is disposed to receive the optical output of themodelocked fiber oscillator prior to reaching the amplifier. Thespectral filter has a spectral transmission with a band edge thatoverlaps the spectral power distribution of the optical output of themodelocked fiber oscillator to attenuate a portion of the spectral powerdistribution and thereby reduce the spectral bandwidth. The pulse widthof the optical pulses coupled from the mode lock fiber oscillator to thefiber amplifier is thereby reduced.

Another embodiment of the invention comprises a method of producingcompressed optical pulses. In this method, longitudinal modes of a fiberresonant cavity are substantially mode-locked so as to produce a trainof optical pulses having a corresponding spectral power distributionwith a spectral bandwidth. The optical pulses are amplified andcompressed to produce compressed optical pulses. The spectral bandwidthof the spectral power distribution is reduced such that the compressedoptical pulses have a shorter duration.

Another embodiment of the invention comprises a pulsed fiber lasercomprising a modelocked fiber oscillator, an amplifier, one or moreoptical pump sources, a pulse compressor, and a pre-compressor. Themodelocked fiber oscillator comprises a gain fiber and a pair ofreflective optical elements disposed with respect to the gain fiber toform a resonant cavity. The modelocked fiber oscillator produces a trainof optical pulses having an average pulse width. The amplifier isoptically connected to the modelocked fiber amplifier such that theoptical pulses can propagate through the amplifier. The fiber amplifieramplifies the optical pulses. The one or more optical pump sources areoptically connected to the modelocked fiber oscillator and the fiberamplifier to pump the fiber oscillator and fiber amplifier. The pulsecompressor is optically coupled to receive the amplified optical pulsesoutput from fiber amplifier. The pulse compressor shortens the pulsewidth of the optical pulses output by the fiber amplifier. Thepre-compressor is disposed in an optical path between the modelockedfiber oscillator and the fiber amplifier. The pre-compressor shortensthe duration of the optical pulses introduced into the fiber amplifiersuch that the pulse duration of the optical pulses output by thecompressor can be further shortened.

Another embodiment of the invention comprises a method of generatingshort high power optical pulses. The method comprises substantiallymode-locking optical modes of a laser cavity to produce an opticalsignal comprising a plurality of laser pulses having an average pulsewidth. The optical signal comprises a distribution of frequencycomponents. The method further comprises compressing the optical pulsesand amplifying the compressed optical pulses to produce amplifiedcompressed optical pulses. The amplified compressed optical pulses arefurther compressed subsequent to the amplifying using a dispersiveoptical element to differentiate between spectral components andintroducing different phase shifts to the different spectral components.

Another embodiment of the invention comprises a pulsed fiber lasercomprising a modelocked fiber oscillator, a fiber amplifier, an opticalpump source, and a pulse compressor. The modelocked fiber oscillatoroutputs optical pulses. The fiber amplifier is optically connected tothe modelocked fiber oscillator and amplifies the optical pulses. Theoptical pump source is optically connected to the fiber amplifier. Thepulse compressor is optically coupled to receive the amplified opticalpulses output from fiber amplifier. The pulsed fiber laser furthercomprises at least one of (i) a first optical tap in the optical pathbetween the modelocked fiber oscillator and the fiber amplifier and afirst feedback loop from the first tap to control the modelocked fiberoscillator based on measurement of output from the first optical tap,and (ii) a second optical tap in the optical path between the fiberamplifier and the compressor and a second feedback loop from the secondtap to control the fiber amplifier based on measurement of output fromthe second optical tap.

Another embodiment of the invention comprises a pulsed light sourcecomprising a light source module, an isolator module, an amplifiermodule, and a compressor module. The light source module comprises anoptical fiber and outputs optical pulses. The isolator module comprisesan optical isolator in a housing having input and output fibers. Theinput fiber is optically coupled to the optical fiber of the lightsource module. The optical isolator is disposed in an optical pathconnecting the input and output fibers such that the optical pulsesintroduced into the input fiber are received by the isolator andpermitted to continue along the optical path to the output coupler. Theamplifier module comprises an amplifying medium and has an optical inputoptically connected to the output fiber of the isolator module toamplify the optical pulses. The compressor module is optically coupledto the amplifier module to compress the optical pulses.

The present invention is directed to a mass-producible passivelymodelocked fiber laser. By incorporating apodized fiber Bragg gratings,integrated fiber polarizers and concatenated sections ofpolarization-maintaining and non-polarization-maintaining fibers, afiber pig-tailed, linearly polarized output can be readily obtained fromthe laser. By further matching the dispersion value of the fiber Bragggrating to the inverse, or negative, of the dispersion of theintra-cavity fiber, the generation of optimally short pulses with alarge optical bandwidth can be induced. In this regard, either positivedispersion fiber in conjunction with negative dispersion fiber gratingsor negative dispersion fiber in conjunction with positive dispersionfiber gratings can be implemented. Preferably, the dispersioncharacteristics of the fiber Bragg grating and the dispersioncharacteristics of the rest of the intra-cavity elements are matched towithin a factor of three. Even more preferably, these characteristicsare matched within a factor of two, or within a factor in the range of1.0 to 2.0. Also preferably, the Bragg grating has a chirp rate greaterthan 80 nm/cm. More preferably, the Bragg grating has a chirp rategreater than 160 nm/cm. Most preferably, the Bragg grating has a chirprater greater than 300 nm/cm. To maximize the output power and the pulserepetition rate, the use of wide-bandwidth fiber Bragg gratings with lowabsolute dispersion is preferable. These fiber Bragg gratings are alsoused as end-mirrors for the cavity and allow the transmission of pumplight to the intra-cavity gain fiber. The fiber Bragg gratings areconveniently produced using phase masks.

Alternatively, fiber couplers can be used inside the fiber cavity.Generally, sections of polarization-maintaining andnon-polarization-maintaining fiber can be concatenated inside the fibercavity. The non-polarization-maintaining section should then be shortenough so as not to excessively perturb the polarization state.Intra-cavity sections of non-polarization-maintaining fiber preferablycomprise all-fiber polarizers to lead to preferential oscillation of onelinear polarization state inside the cavity. Similarly, when directlyconcatenating polarization-maintaining fiber sections, the length of theindividual section should be long enough to prevent coherentinteractions of pulses propagating along the two polarization axes ofthe polarization-maintaining fibers, thereby ensuring a maximum in pulsestability.

Saturable absorber mirrors (SAMs) placed inside the cavity enablepassive modelocking. The saturable absorbers (SA) can be made frommultiple quantum wells (MQW) or bulk semiconductor films. Thesesaturable absorbers have preferably a bi-temporal life-time with a slowcomponent (>>100 ps) and a fast component (<<20 ps). The realization ofthe bi-temporal dynamics of the optical nonlinearity is achieved bytailoring the depth profile of the ion-implantation in combination withthe implantation dose and energy. The result is that the carriers trapat distinctively different rates in different depth regions of the SAM.

Saturating semiconductor films can for example be grown fromaluminum-containing material such as AlGaInAs, the exact composition canbe selected depending on the sought band-gap (typically selected to bein the vicinity of the desired operating wavelength of the laser system)and it is also governed by the requirement of lattice-match between thesaturating semiconductor film and an underlying Bragg mirror or anyother adjacent semiconductor material. Compositional requirementsenabling lattice match between semiconductors and/or a certain bandgapare well known in the state of the art and are not further explainedhere.

In aluminum containing semiconductors the surface area can induce a lowoptical damage threshold triggered by oxidization of the surface. Inorder to prevent optical damage of aluminum containing surface areas apassivation layer, e.g., InP, InGaAs or GaAs, is incorporated. SAdegradation is further minimized by optimizing the optical beam diameterthat impinges on the SAM. In one implementation the SAM and anintra-cavity fiber end can be either butt-coupled or brought into closecontact to induce modelocking. Here, the incorporation of a precisionAR-coating on the intra-cavity fiber end minimizes any bandwidthrestrictions from etalon formation between the SAM and the fiber end.Etalons can also be minimized by appropriate wedging of the fiber ends.The beam diameter inside the SAM can be adjusted by implementing fiberends with thermally expanded cores. Alternatively, focusing lenses canbe directly fused to the fiber end. Moreover, graded-index lenses can beused for optimization of the focal size and working distance between thefiber tip and SA surface.

Wavelength tuning of the fiber lasers can be obtained by heating,compression or stretching of fiber Bragg gratings or by theincorporation of bulk optic tuning elements.

The use of bi- or multi-temporal saturable absorbers allows the designof dispersion compensated fiber laser operating in a single-polarizationstate, producing pulses at the bandwidth limit of the fiber gain medium.Additional spectral broadening can be obtained by launching these pulsesinto highly nonlinear fibers, allowing for the generation ofbroad-bandwidth pulses with bandwidths exceeding one octave for use inoptical coherence tomography or in precision metrology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a generalized modular approach to forming a highpower laser pulse;

FIG. 2A schematically illustrates one embodiment of a fiber basedoscillator;

FIG. 2B schematically illustrates one embodiment of a fiber basedoscillator having a filter inside the cavity to remove dispersive waveside-peaks in a soliton oscillator;

FIG. 2C illustrates an exemplary optical spectrum output by theoscillator;

FIGS. 3A-C illustrate various exemplary combinations and arrangements ofmodular components of a laser for generating a high power short pulseoutput;

FIGS. 4A-C illustrate the use of a pre-compressor to shorten the seedpulse width prior to amplification in the exemplary modular lasers;

FIGS. 5A-C illustrate the use of a photonic crystal fiber as apre-compressor;

FIGS. 6A-C illustrate the use of a fiber Bragg grating as apre-compressor;

FIGS. 7A-C illustrate the use of a band-pass filter to select a part ofthe oscillator output spectrum and shorten the seed pulse prior toamplification;

FIG. 8 illustrates an exemplary sub-100 femtosecond (fs) pulse generatedwith use of the band-pass filter upstream of the amplifier;

FIGS. 9A-C illustrate the use of a long period fiber grating to select apart of the oscillator output spectrum and shorten the seed pulse priorto amplification;

FIG. 10 illustrates a exemplary laser system having one or more tap andfeedback components to monitor the performance of the system andactively control the system for stable operation;

FIGS. 11A, B and C illustrate exemplary embodiments of the engineeredmodules including a saturable absorber module, an attenuator module andan isolator module;

FIG. 11D illustrates one embodiment of the oscillator having one or moremodular sub-components with a temperature control component;

FIG. 12 illustrates how two exemplary components or modules can beoptically coupled by splicing of fiber pigtails; and

FIG. 13 illustrates some of possible advantages of flexibility inarrangement of modular components provided by the spliced opticalcoupling.

FIG. 14 is a diagram of a cladding pumped fiber cavity design accordingto a first embodiment of the invention.

FIG. 15 a is a diagram of a saturable absorber mirror according to anembodiment of the invention.

FIG. 15 b is a diagram of a saturable absorber mirror according to analternative embodiment of the invention.

FIG. 16 is a diagram of the proton concentration as a function of depthobtained after proton implantation into a saturable semiconductor film.

FIG. 17 is a diagram of the measured bi-temporal reflectivity modulationobtained in a semiconductor saturable mirror produced byion-implantation with selective depth penetration.

FIG. 18 a is a diagram of a scheme for coupling a saturable absorbermirror to a fiber end according to an embodiment of the invention.

FIG. 18 b is a diagram of a scheme for coupling a saturable absorbermirror to a fiber end according to an alternative embodiment of theinvention.

FIG. 19 is a diagram for increasing the optical bandwidth of a fiberlaser according to an embodiment of the invention.

FIG. 20 is a diagram of a core pumped fiber cavity design according toan embodiment of the invention.

FIG. 21 is a diagram of a core pumped fiber cavity design usingintra-cavity wavelength division multiplexers and output couplersaccording to an embodiment of the invention.

FIG. 22 is a diagram of a core pumped fiber cavity design usingintra-cavity wavelength division multiplexers and a butt-coupled fiberpig-tail for output coupling according to an embodiment of theinvention.

FIG. 23 is a diagram of a cladding pumped fiber cavity design using anintra-cavity output coupler according to an embodiment of the invention.

FIG. 24 is a diagram of a cladding pumped fiber cavity design usingintra-cavity fiber output couplers according to an embodiment of theinvention.

FIG. 25 a is a diagram of a passively modelocked fiber laser based onconcatenated sections of polarization maintaining and non-polarizationmaintaining fiber sections according to an embodiment of this invention.

FIG. 25 b is a diagram of a passively modelocked fiber laser based onconcatenated sections of long polarization maintaining fiber sectionsaccording to an embodiment of this invention.

FIG. 25 c is a diagram of a passively modelocked fiber laser based onshort concatenated sections of polarization maintaining fiber andadditional sections of all-fiber polarizer according to an embodiment ofthis invention.

FIG. 26 is a diagram of a dispersion compensated fiber laser cavityaccording to an embodiment of this invention.

FIG. 27 is a diagram of a dispersion compensated fiber laser cavityaccording to an alternative embodiment of this invention, includingmeans for additional spectral broadening of the fiber laser output.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

These and other aspects, advantages, and novel features of the presentteachings will become apparent from the following detailed descriptionand with reference to the accompanying drawings. In the drawings,similar elements have similar reference numerals.

FIG. 1 illustrates an overall concept of a pulse laser system 100 thatoutputs a high power short pulse 116. Preferably, the output pulse 116has a temporal width that is less than approximately 200 femtoseconds(fs), and an average power that is greater than approximately 200milliwatts (mW). It will be noted that these exemplary performanceparameters are in no way intended to limit the scope of the presentteachings.

To achieve a clean short pulse, several techniques can be included suchas for example inclusion of attenuators, spectral filters, andcompression elements as discussed more fully below. Use of one or moreof such components can provide pulse widths of about 90 fs or less andaverage power of 200 mW or more.

Another aspect of the present teachings relates to a modular designapproach where various components can be packaged as modules and themodular components can then be connected as needed. Some modules maycomprise optical elements such as bulk optics or planar waveguidespackaged in a housing that shields various optical elements from theoperating environment. Fiber pigtails may extend from these housing,which may include thermal insulation and may be hermetically sealed. Thefiber pigtails, potentially enable seamless connection to other modulesvia, e.g., optical fiber fusion splices. Packaging in such casing may beparticularly advantageous for modules containing bulk or physical opticsand opto-mechanical elements, in contrast to fiber elements, which maynot require such encasement.

Such a system using fiber based components or packages having fiberinputs and outputs can benefit from the compact nature of thecomponents, as well as effective and compact coupling afforded bysplicing of the fibers. The optics within the modules preferablycomprise micro-optics and fiber optics or other waveguide elements.Accordingly, the modules may be small and have reduced form factor. Themodular approach may also simplify repair and alteration of lasersystems as the modules can be readily substituted or replaced andre-spliced into place in the laser system.

As shown in FIG. 1, an exemplary laser system 100 comprises a seed pulsegenerator 102 optically coupled to a pulse conditioner 104 by a coupling120. The seed pulse generator 102 provides a seed pulse 110 to the pulseconditioner 104. One way to amplify a short pulse in an amplifier is tobroaden the pulse, lowering the amplitude of the pulse feed to theamplifier. Such an amplitude-lowered pulse can then be amplified toincrease the amplitude, preferably within the linearity region of theamplifier. The amplified pulse having a broadened width can then becompressed to yield a relatively high amplitude and relatively shortpulse output.

In FIG. 1, the pulse conditioner 104 is depicted as broadening its inputseed pulse 110 to yield a broad and low amplitude pulse 112. The lasersystem 100 further comprises an amplifier 106 optically coupled to theconditioner 104 by a coupling 122. The amplifier 106 is depicted asamplifying the amplitude of the broadened pulse 112 to yield anamplified broad pulse 114. The amplified broad pulse 114 is depicted asbeing compressed by a compressor 108 (that is coupled to the amplifier106 by a coupling 124) to yield an amplified short pulse output 116 asan output 126. This compressor 108 may be excluded in certain cases inembodiments described below.

It will be understood that this simplified description of pulseamplification is exemplary of a general process of amplifying a shortpulse. It will also be understood, and as described below, that pulseconditioning can involve optical operations other than temporallystretching of the seed pulse. Other variations can be incorporated intothe laser system to accommodate various design goals and operatingconditions. Some of such design considerations are described below ingreater detail. In some embodiments, for example, the pulse conditioner104 and/or compressor 108 may be excluded. Other variations in theconfiguration and implementation of the laser system 100 are alsopossible.

In some embodiments, the seed pulse generator 102 comprises anoscillator having a rare earth doped fiber. Dopants may include, forexample, Er, Yb, Nd or combinations thereof as well as other materials.The doped fiber can be single clad or double clad and may bepolarization maintaining or non-polarization maintaining. Both activeand passive modelocking techniques can be used to generate short andultra-short pulses in the rare-earth doped fiber, with the passive onesimpler and intrinsically more stable. Three common passive mode-lockingtechniques involve a saturable absorber being part of the cavity,nonlinear polarization evolution, or a combination thereof. In certainapplications, passive modelocking techniques based on saturableabsorbers are preferred and permit the construction of relatively simpleand reliable cavities. Additional details regarding passive mode-lockingtechniques are disclosed in a copending U.S. patent application Ser. No.10/627,069 filed on Jul. 25, 2003, by M. E. Fermann, and G. C. Choentitled “Polarization Maintaining Dispersion Controlled Fiber LaserSource Of Ultrashort Pulses” (Docket No. IMRAA.021A/IM-99), which ishereby incorporated herein by reference in its entirety.

The amplifier may comprise a fiber amplifier having a gain fiber such asa doped fiber. The amplifier, however, should not be limited to fiberamplifiers. Similarly, the amplifier may comprise a parabolic pulseamplifier as described in copending U.S. patent application Ser. No.09/576,772 filed May 23, 2000, by M. E. Fermann, A. Galvanauskas, and D.Harter entitled “Modular, high energy, widely-tunable ultrafast fibersource” (Docket No. IM-83), which is hereby incorporated herein byreference in its entirety. Other types of amplifiers, however, may beemployed as well.

FIG. 2A illustrates one embodiment of a fiber-based oscillator 130 thatcan provide a passive modelocked seed pulse. The exemplary oscillator130 comprises a saturable absorber 132 and a Yb-doped gain fiber 140that is pumped by a pump diode 148 via a pump coupler 146. (Yb-dopedfiber is a good candidate, for example, in the spectral range from 1.0μm to 1.1 μm because Yb ions present a large absorption cross sectionnear 980 nm which allows to be pumped with low-cost commerciallyavailable laser diodes. In addition, the large fluorescence spectralrange of this fiber enables the short pulse generation.) The pump diode148 shown in FIG. 2A may be a part of the integrated oscillator module,or may be a separate module that provides an input to the oscillatormodule. The oscillator 130 further comprises an output fiber 144 coupledfrom the gain fiber 140 via a fiber grating 142. The fiber grating 142can function as a dispersion controlling element and at the same timeserve as an output coupler.

In one embodiment, the oscillator cavity fiber comprises a section ofYb-doped polarization maintaining gain fiber. The oscillator cavityfiber can further comprise an undoped polarization maintaining fibersection for controlling the total intracavity dispersion. In someembodiments, the length of this undoped portion is selected such thatthe undoped portion of fiber together in combination with doped fiberand the chirped fiber Bragg grating (one embodiment of the fiber grating142) provides a zero or negative total dispersion in the cavity.

In one embodiment, the cavity fiber is relatively shorter. The use of ashorter gain fiber 140 is typically associated with a high pumping rate,thus driving the gain dynamics closer to a saturation level than thatassociated with a longer fiber. In addition, the population variation inthe ground state of Yb dopants becomes less susceptible to theenvironmental temperature variation. These effects enhance theoperational stability of the oscillator 130 that is exposed to variationof environmental temperature. In one embodiment, oscillator outputstability is demonstrated by a dependency of output on environmentalfiber temperature that is less than approximately 0.5%/C, presuming thatother modules are kept at a substantially constant temperature.

As shown in FIG. 2A, the exemplary oscillator 130 further comprises anassembly of optical elements that optically couples the saturableabsorber 132 to the gain fiber 140. In one embodiment, the assembly ofoptical elements comprises lenses 134 and 138 that collimate and focusthe light between the fiber 140 and the saturable absorber 132. Theassembly can further comprise a polarizer 136. These components can beinclude in a housing with a fiber pigtail as discussed more fully inconnection with FIG. 11A.

One embodiment of the exemplary fiber oscillator in FIG. 2A can generatepulses with a spectral bandwidth in the range of approximately 1 to 30nm, depending on the dispersion parameter of the fiber Bragg grating 142used. With a high negative dispersion fiber Bragg grating 142, a narrowbandwidth pulse can be generated. Side peaks can also be generated duedispersive wave shedding by the soliton in the oscillator. Such sidepeaks can be substantially removed by including a bandpass filter.

FIG. 2B illustrates one embodiment of an oscillator 150 that includes abandpass filter 165 inside the cavity. Such a filter can remove the sidepeaks associated with the high negative dispersion fiber Bragg gratingdescribed above in reference to FIG. 2A, while maintaining asubstantially similar spectral bandwidth.

In one embodiment, the bandwidth of the filter 165 can be predeterminedwithin approximately 1-2 nanometer accuracy by analytical orexperimental analysis of the system. Another method is to use arotatable dielectric bandpass filter and rotate the filter to adifferent incidence angle and utilize the associated etalon effect toprovide a variation of the spectral position and width. Yet anothermethod is to modify the spectral shape of the transmission, for exampleusing v- or u-type coating used in the dielectric coating industry. Sucha filter can substantially eliminate side lobes in the spectrum, whichmay originate from nonlinear phase distortion at excessive gain or fromhigh-order soliton formation.

As shown in FIG. 2B, the exemplary oscillator 150 having the bandpassfilter 165 is depicted as being generally similar in design to theexemplary oscillator 130 of FIG. 2A. Saturable absorber 152, lenses 154,158, polarizer 156, gain fiber 160, fiber grating 162, output fiber 164,pump coupler 166, and pump diode 168 can correspond to the saturableabsorber 132, lenses 134, 138, polarizer 136, gain fiber 140, fibergrating 142, output fiber 144, pump coupler 146, and pump diode 148 ofthe oscillator 130. The oscillator 150 is illustrated for the purpose ofdescribing the possible use of the bandpass filter 165. Thus, it will beunderstood that such a similarity in two exemplary embodiments of theoscillators should in no way be construed to limit the design of theoscillator to such a configuration.

As described above in reference to FIG. 2B, the high negative dispersionfiber grating and the associated side lobe effect can be mitigated bythe use of the bandwidth filter. When a low negative dispersion gratingis used, the oscillator (e.g., 130 in FIG. 2A) can generate largebandwidth spectrum, and substantially no side peak is observed.

FIG. 2C illustrates an exemplary spectrum 170 generated by theoscillator described above in reference to FIG. 2A, where the fiberBragg grating has a dispersion of approximately −0.11 ps². The spectralbandwidth of this oscillator output is around 19 nm.

It will be appreciated that although the oscillator (130 and 150 inFIGS. 2A and B) is described as a fiber oscillator, the concepts of thepresent teachings are not limited to such oscillators. The modularoscillator can be any type of a pulsed laser preferably that outputspulses with a temporal width preferably less than approximately 500 ps.The spectral bandwidth of the seed output from the oscillator ispreferably greater than or equal to about 8 to 10 nm, although thebandwidth may be outside this range. As described herein, however, thefiber-based laser is preferred for compact packaging reasons. Otherwaveguide-based lasers are also possible and may yield compact designsas well.

As described more fully below, the oscillator 130, 150 or portionsthereof may be packaged in a housing that provides a substantiallystable support for optical elements. Such a housing preferably offersprotection from the environment and improves performance stability ofthe optical devices. The packages may include fiber pigtail inputsand/or fiber pigtail outputs, which may be connected to othercomponents. These pigtails may comprise single mode polarizationmaintaining fiber although other types of pigtail fibers may beemployed. Some of the components in the laser system may also compriseoptical fiber or fiber components that are not enclosed in a housing butthat are spliced to the fiber pigtails.

FIGS. 3-9 now illustrate block diagrams of various possible ways ofconditioning the seed pulse generated by the oscillator module. FIGS.3A-C illustrate three exemplary embodiments of a basic modular designfor producing high power and short pulses. As shown in FIG. 3A, oneembodiment of a laser system 180 comprises an oscillator 182 coupled toan amplifier 186 via an isolator 184. The isolator 184 between theoscillator 182 and the amplifier 186 may comprise an independent modularcomponent, or may include the fiber Bragg grating described above inreference to FIGS. 2A and B. In the latter case, at least a portion ofthe functional blocks of oscillator 182 and the isolator 184 may bephysically packaged in a same module. The inclusion of the isolator isto provide a barrier to block any back reflection from the downstreamback to the upstream such as oscillator or amplifier so as to maintain astable operation, see FIG. 11C for detail. The output of the amplifier186 is also coupled to a compressor module 190 via an isolator 188.

FIG. 3B illustrates another embodiment of a basic design of a lasersystem 200 comprising an oscillator 182 and an amplifier 206 with anattenuator 202 therebetween. This laser system 200 further comprises anisolator 204 between the oscillator and the amplifier. The attenuator202 is disposed after the isolator 204 and before of the amplifier 206.

FIG. 3C illustrates another embodiment of a basic design of a lasersystem 220 also comprising an oscillator and an amplifier with anattenuator 222 therebetween. The attenuator 222 is disposed after theoscillator 224 and before the isolator 226.

Preferably the attenuator is a variable attenuator and has a variabletransmission that can be controlled in the range of about 1˜20 dB. Suchvariable attenuation can be advantageously employed in the manufactureand tuning of the laser systems.

Fiber amplifiers exhibit a variation in performance from to unit tounit. As a result, different amplifiers that are incorporated into thelasers during manufacturing may require different amplitude input pulsesto provide similarly operating lasers. To accommodate such variation inamplifier performance, an adjustable attenuator such as shown in FIGS.3B-3C may be included in the laser system. During fabrication of thelaser, the components may be tested and the attenuator adjusted toprovide a suitable amplitude for the optical pulse to the amplifier. Insuch a manner, substantially standardized laser performance can beachieved for a particular product.

Different measurements and analysis of these measurements may beemployed to determine the suitable adjustment to the variableattenuator. For example, pulse power and/or pulse width may be measuredor spectral measurements may be employed. Measurements may be obtainedbefore or after the attenuator or at the output of the amplifier or thelaser, or elsewhere. Other measurements may be used as well and likewisethe measuring and evaluation techniques should not be limited to thoserecited herein. The attenuator, however, can be adjusted in response tosuch measurements

This variable attenuator may comprise a rotatable waveplate and apolarization beamsplitter such as a MacNeille polarizer. The waveplatecan be rotated to vary the distribution of light into orthogonalpolarizations. The polarization beamsplitter can be used to direct aportion of the light out of the laser, depending on the state of thewaveplate. Accordingly, a user, by rotating the waveplate and alteringthe polarization of light can control the amount of light reaching theamplifier and thereby adjust the system. Preferably, the waveplate andthe MacNeille polarizer comprise micro-optics or are sufficiently smallto provide for a compact laser system. These optical elements are alsopreferably packaged in a housing that may include fiber pigtailconnections as shown in FIG. 11B. Lenses, mirrors, or other couplingelements may be employed to couple light from and to the pigtail inputsand outputs. The optics, e.g., waveplate, MacNeille polarizer, couplinglens, mirrors, etc., may be mounted to the housing to provide alignmentand support. The housing may also provide protection from theenvironment and may be thermally insulating and possibly hermeticallysealed. Preferably, however, the module formed by the optics and thehousing are compact and rugged.

Other types of attenuator modules may alternately be employed. Variableattenuation need not be provided by a waveplate and a polarizationselective optical element. Fiber or waveguide elements may be used aswell. Such variable attenuation may be controlled manually orautomatically. Various other designs are possible.

The system configuration may be varied as well. Pulse stretching neednot always be incorporated in the laser system. For example, in a fiberamplifier system with sub-microjoule pulse energy, a few picosecond ofseed pulse can be sufficiently long to reduce the nonlinear phasemodulation in the amplifier fiber significantly. Thus, employment of apulse stretcher in form of a long length of fiber or chirped fiber Bragggrating in generally may not necessarily be required in such a case.

In general, the preconditioning of the seed pulse property in thetime-domain alone, e.g., stretching pulse width, is not always asuitable method for generating high quality amplified pulses. In avariety of cases, manipulation of the seed pulse by the seed pulsegenerator may readily provide sufficient degree of freedom for pulserecompression The outcome may not necessarily be the most preferred,however, depending on the target property of the recompressed pulses,such an approach significantly simplifies the technical complexity ofthe pulse controlling method.

FIGS. 4A-C illustrate how an input seed to an amplifier can be furtherconditioned to improve the manner in which the laser system operates.For the purpose of description, the exemplary designs shown in FIGS.4A-C build on the exemplary basic designs described above in referenceto FIGS. 3A-C.

In general, the output pulse from an oscillator is delivered to anamplifier for higher power. As a result of the delivery fiber(s) betweenthe oscillator and the amplifier, the pulse width may be stretched. Forexample, approximately 2 meters of delivery fiber may correspond toapproximately 1 ps pulse width increase. A preferred seed pulse width,however, can be characterized as being less than approximately 1 ps.

To obtain a clean and shorter pulse after a compressor, severaltechniques can be used to preferably make the seed pulse width shorterprior to reaching the amplifier. One method is to pre-compress the seedpulse before injecting it into the amplifier. In one embodiment, use ofa pre-compressing component can yield a pre-compressed seed pulse havinga pulse width less than about 1 ps and preferably as low asapproximately 150 fs. Such a pre-compressed seed pulse fed into theamplifier can yield an amplifier output having a spectral bandwidth thatis greater than approximately 30 nm due to nonlinear effect. Such anamplifier output can be compressed by a compressor to yield a finalpulse having a width of approximately 100 fs.

As shown in FIG. 4A, one embodiment of an exemplary laser system 240 issimilar to the exemplary basic design of FIG. 3A. A modularpre-compressor component 242 is disposed in an optical path between anisolator 244 and an amplifier 246. The pre-compressor 242 pre-compressespulses being output by the isolator 244 prior to being injected into theamplifier 246.

FIG. 4B illustrates one embodiment of an exemplary laser system 260 thatis similar to the exemplary basic system of FIG. 3B and that includes anattenuator 264. A modular pre-compressor component 262 is disposed in anoptical path between the attenuator 264 and an amplifier 266. Thepre-compressor 262 pre-compresses pulses being output by the attenuator264 prior to being injected into the amplifier 266.

FIG. 4C illustrates one embodiment of an exemplary laser system 280 thatis similar to the exemplary basic system of FIG. 3C. A modularpre-compressor component 282 is disposed in an optical path between anisolator 284 and an amplifier 286. The pre-compressor 282 pre-compressespulses being output by the isolator 284 prior to being injected into theamplifier 286.

The pre-compressor may comprise bulk, fiber, or other waveguide optics.Examples of components that may comprise the pre-compressor moduleinclude a bulk grating pair, a single grating (e.g., bulk or waveguide),a prism pair, etc. Fiber components such as chirped fiber Bragg gratingsmay also be employed. Other fiber and non-fiber components may beemployed as well. In some embodiments, the components are encapsulatedin a housing that provides for substantially stable mounting therein, aswell as protection from the environment. The housing may furthercomprises a fiber pigtail input and/or a fiber pigtail output. Suchpigtail fibers can be spliced in a manner described above.

FIGS. 5A-C now illustrate the use of a photonic crystal fiber as apre-compressor. Photonic crystal fibers may be tailored to control theirdispersion and to provide negative dispersion at the operatingwavelength. In one embodiment, a short piece of a photonic crystal canresult in the injection seed pulse (into the amplifier) being compressedto a femtosecond regime. Advantageously, the photonic crystal fiber, dueto its fiber nature, can be incorporated conveniently into a lasersystem having other fiber based components such as by splicing,providing a seamless connection in a compact and rugged configuration.

FIG. 5A illustrates one embodiment of an exemplary laser system 300 thatis similar to the exemplary laser system of FIG. 4A. The pre-compressorcomprises a photonic crystal fiber 302 that is disposed in an opticalpath between the isolator and the amplifier.

FIG. 5B illustrates one embodiment of an exemplary laser system 320 thatis similar to the exemplary laser system of FIG. 4B and includes anattenuator. The pre-compressor comprises a photonic crystal fiber 322that is disposed in an optical path between the attenuator and theamplifier.

FIG. 5C illustrates one embodiment of an exemplary laser system 340 thatis similar to the exemplary laser system of FIG. 4C and also includes anattenuator. The pre-compressor, however, comprises a photonic crystalfiber 342 that is disposed in an optical path between the isolator andthe amplifier.

FIGS. 6A-C illustrate the use of a fiber Bragg grating as apre-compressor. Advantageously, the fiber Bragg grating, due to itsfiber nature, can be incorporated conveniently into a laser systemhaving other fiber based components.

FIG. 6A illustrates one embodiment of an exemplary laser system 360 thatis similar to the exemplary laser system of FIG. 4A. The pre-compressorcomprises a fiber Bragg grating 362 that is disposed in an optical pathbetween the isolator and the amplifier.

FIG. 6B illustrates one embodiment of an exemplary laser system 380 thatis similar to the exemplary laser system of FIG. 4B and comprises anattenuator. The pre-compressor comprises a fiber Bragg grating 382 thatis disposed in an optical path between the attenuator and the amplifier.

FIG. 6C illustrates one embodiment of an exemplary laser system 400 thatis similar to the exemplary laser system of FIG. 4C and also comprisesan attenuator. The pre-compressor comprises a fiber Bragg grating 402that is disposed in an optical path between the isolator and theamplifier.

The nonlinearity in the amplifier can be a result of an interplay ofdifferent factors, such as gain shaping, inhomogeneous self-phasemodulation, and higher order dispersion. Such factors can be highlysensitive to the spectral position within the whole pulse spectrum. Oneway to control these factors and thereby control the nonlinearity in theamplifier is to manipulate the spectrum being output from theoscillator. In one embodiment, a bandpass filter is used to select aportion of the oscillator spectrum to be amplified. At the same time,the filter can shorten the pulse width of the seed.

In various embodiments, bandpass, highpass, or lowpass filters may beemployed to spectrally narrow and control a spectral power distributionof an optical output from the oscillator. Preferably, the filter hasspectral transmission with a band edge that overlaps the spectral powerdistribution of the output pulses from the oscillator. The bandpassfilter thereby attenuates a portion of the spectral power distributionand reduces the spectral bandwidth. The pulse width of the opticalpulses coupled from the oscillator to the fiber amplifier are therebyreduced. Preferably, the resultant spectral bandwidth is between about 5and 12 nm and preferably less than about 10 nm but may be outside thisrange.

The spectral filter may comprise, for example, a fiber or otherwaveguide devices such as a planar waveguide element or may comprisebulk optics. Examples of spectral filters include gratings, etalons,thin film coatings, etc. Preferably, the filter comprises micro-optics.In various preferred embodiments, the filter comprises a fiber elementssuch as a fiber Bragg grating that can be readily physically connectedto a fiber based system in a compact robust manner. Other types offilters and configurations for providing spectral filtering may also beemployed.

FIGS. 7A-C illustrate the use of a bandpass filter as a spectrumselector/pre-compressor for the exemplary designs similar to that ofFIGS. 4A-C.

FIG. 7A illustrates one embodiment of an exemplary laser system 420 thatis similar to the exemplary laser system of FIG. 4A. The spectrumselector/pre-compressor comprises a bandpass filter 422 that is disposedin an optical path between the isolator and the amplifier.

FIG. 7B illustrates one embodiment of an exemplary laser system 440 thatis similar to the exemplary laser system of FIG. 4B and that includes anattenuator. The spectrum selector/pre-compressor comprises a bandpassfilter 442 that is disposed in an optical path between the attenuatorand the amplifier.

FIG. 7C illustrates one embodiment of an exemplary laser system 460 thatis similar to the exemplary laser system of FIG. 4C and also includes anattenuator. The spectrum selector/pre-compressor comprises a bandpassfilter 462 that is disposed in an optical path between the isolator andthe amplifier.

The position of the spectral filter is not limited to the locationsshown herein. Preferably, however, the spectral filter is disposed in anoptical path between the oscillator and the amplifier.

FIG. 8 illustrates an exemplary pulse 480 generated by a laser systemusing the bandpass filter described above in reference to FIGS. 7A-C. Inone embodiment, the oscillator output has a bandwidth of approximately12 nm. Halving the seed spectral bandwidth from approximately 12 nm toapproximately 6 nm with a bandpass filter can result in the selectedpulse width also being halved from approximately 1.3 ps to approximately700 fs. By using a bandpass filter with a narrow bandwidth or a tiltfilter, even shorter pulses can be achieved without compressor elements.The exemplary pulse 480 output from the compressor, has a width ofapproximately 88 fs.

As an alternative to a free space bandpass filter, a fiber band passfilter such as a long-period fiber grating can be used to select a partof the oscillator output signal in both wavelength domain and timedomain. A long-period fiber grating (LPG) couples the light from afundamental guided mode to forward-propagating cladding modes. A pair ofmatched LPGs can be used. One LPG couples light from the fundamentalmode to the cladding mode and one LPG couples light back from thecladding mode to the fundamental mode. The cladding can thereby be usedas a bypass for the resonant light while the non-resonant lightpropagating in the core is substantially blocked. Alternatively, byintroducing a n-phase shift in the approximate middle of a LPG duringits fabrication, a bandpass filter can be made using only one LPG. Then-phase shift in the LPG reverses the coupling direction such that lightcoupled into the cladding mode can return into the fundamental mode. Oneadvantage of using an LPG as a bandpass filter is that the transmissionspectrum can be conveniently designed so as to be suitable for a givenamplifier operation.

FIGS. 9A-C illustrate the use of a long-period fiber grating (LPG) as aspectrum selector/pre-compressor for the exemplary designs similar tothat of FIGS. 4A-C.

FIG. 9A illustrates one embodiment of an exemplary laser system 490 thatis similar to the exemplary laser system of FIG. 4A. The spectrumselector/pre-compressor comprises an LPG 492 that is disposed in anoptical path between the isolator and the amplifier.

FIG. 9B illustrates one embodiment of an exemplary laser system 510 thatis similar to the exemplary laser system of FIG. 4B and that comprisesan attenuator. The spectrum selector/pre-compressor comprises an LPG 512that is disposed in an optical path between the attenuator and theamplifier.

FIG. 9C illustrates one embodiment of an exemplary laser system 530 thatis similar to the exemplary laser system of FIG. 4C and that alsocomprises an attenuator. The spectrum selector/pre-compressor, however,comprises an LPG 532 that is disposed in an optical path between theisolator and the amplifier.

The spectral manipulation can be achieved with a spectral filter thatprovides a band selectivity and/or a spectral shape modification. Theband selectivity can provide a proper selection of the seed spectrum(e.g. position and bandwidth). The position may range from about 1045 to1055 nm and the bandwidth may range from about 5 to 12 nm in certainembodiments although these ranges should not be construed as limiting asother embodiments are possible. The selected band may be matched withthe gain shaping and nonlinear phase distortion in the amplifier for animproved pulse compressibility. The proper spectral shape, such asGaussian or flat-top or other profile, can also be tailored with aspectral shaping filter. This concept can be further extended to anactively or passively controlled “pedestal flattening filter”.

FIG. 10 illustrates one embodiment of a laser system 550 having amonitoring and feedback control capability. A rare-earth-doped fiberabsorption depends heavily on the environmental temperature. Such adependence can result in a laser performance drift with temperature. Forexample, the oscillator modelocking threshold typically increases athigh temperature.

In one embodiment of the laser system, monitoring the performance suchas output power at some point(s) of the system and providing feedback tothe diode pump drivers for active control can achieve a stableoperation. FIG. 10 illustrates one embodiment of a laser system 550having such a monitoring and feedback feature. The exemplary lasersystem 550 comprises an oscillator 552 coupled to an attenuator 556 viaan isolator 554. The output from the attenuator 556 is fed into abandpass filter 558 whose output is then direct to an amplifier 560. Theoutput from the amplifier 560 is fed into a compressor 564 via anisolator 562. It should be noted that the use of the attenuator 556 andthe bandpass filter 558 are exemplary, and that either of thesecomponents may be excluded and any other modular components, includingthose disclosed herein, may be used in the laser system having thefeedback.

As shown in FIG. 10, the laser system 550 further comprises a firstmonitor component 570 that monitors a performance parameter of thesystem after the oscillator 552. The monitor 570 may comprise a sensorand controller. The monitor 570 may issue adjustment commands to a firstdriver 572 that implements those adjustment commands at the oscillator552.

The exemplary laser system 550 is shown to further comprise a secondmonitor component 574 that monitors a performance parameter of thesystem after the amplifier 560. The monitor 574 may similarly comprise asensor and controller. The monitor 574 can then issue adjustmentcommands to a second driver 576 that implements those adjustmentcommands at the amplifier 560.

The monitoring of the system performed by the exemplary monitors 570and/or 574 may comprise for example an optical detector and electronicsthat monitors optical intensity or power or other relevant parametersuch as, e.g., frequency and spectrum. In response to such measurement,the monitor and the driver may induce changes in the oscillator and/orthe amplifier by for example adjusting the pump intensity and/or rate,or adjusting the operating temperature. Exemplary embodiments thatinclude temperature control of the oscillator are described more fullybelow. Temperature control of the oscillator can stabilize the gaindynamics as well as frequency fluctuations. Temperature control of theamplifier can also be used to stabilize the gain dynamics.

Other configurations for providing feedback to control the operation ofthe laser system may also be employed. For example, more or lessfeedback loops may be included. The loops may involve electronics thatperform operations such as calculations to determine suitableadjustments to be introduced. The feedback may be obtained from otherlocations in the system and may be used to adjust other components aswell. The embodiments described in connection with FIG. 10 should not beconstrued to limit the possibilities.

In the description of various exemplary designs presented in referenceto FIGS. 3-10, various components are depicted as modules. These modulesmay indeed be independent modular components. Other configurations,however, are possible. For example, two or more of these components maybe packaged together in an integrated module. Alternatively, modulesthat may in some embodiments include one or more optical element, may bebroken-up and/or separated and included in separate modules.

In one embodiment, an isolator depicted, for example, in FIGS. 3A-3C asbeing downstream from an oscillator may comprise a fiber Bragg grating,and may be packaged together with the oscillator. The isolator isthereby merged with the oscillator. Similarly, various components thatmay be considered separate functional groups may be included in the samehousing and support structures. For example, the amplifier may comprisea non-fiber element such as a solid state or planar waveguide amplifierand may be included on the same platform and encased in the same housingas isolator optics thereby merging the amplifier and isolatorfunctionalities. Separation of the functionalities, however, may offeradvantages and thus be preferred in some embodiments

The modular design concept can also be further extended to the elementswithin the oscillator, amplifier, and other components comprising thelaser system. FIGS. 11A-C illustrate exemplary embodiments that can bedesigned and packaged with the advantageous modular approach of thepresent teachings.

FIG. 11A illustrates one embodiment of a saturable absorber module 710packaged following Telcordia specifications. The saturable absorbermodule 710 comprises a housing 712 that contains a plurality of opticalelements. An optical fiber connector 714 comprising an optical fiber 716having an angle polished or cleaved endface passes through one side ofthe housing 712 into an inner region of the housing containing theplurality of optical components. These optical components include afirst lens 718 for collecting and preferably collimating light outputfrom the optical fiber 716, a variable waveplate 720 and a polarizationselective optical element 722 as well as a saturable absorber 724. Thevariable waveplate 720 comprises a rotatable waveplate mounted on arotatable wheel 726 and the polarization selective optical element 722comprises a polarization beamsplitter such as a MacNeille prism. Asecond lens 728 disposed between the polarization selective opticalelement 722 and the saturable absorber 724 preferably focuses lightpropagating through the waveplate 720 and the polarization beamsplitter722 onto the saturable absorber. An optical path is formed from theoptical fiber 716 through the waveplate 720 and prism 722 to thesaturable absorber 724, which is reflective. Light will propagate inboth directions along this optical path, which may form a portion of theresonator of the oscillator.

In various preferred embodiments, the light in the laser is linearlypolarized. The degree of the linear polarization may be expressed by thepolarization extinction ratio (PER), which corresponds to a measure ofthe maximum intensity ratio between two orthogonal polarizationcomponent. In certain embodiments, the polarization state of the sourcelight may be maintained by using polarization-maintaining single-modefiber. For example, the pigtail of the individual modular device may befabricated with a polarization-maintaining fiber pigtail. In such cases,the PER of each modular stage may be higher than about 23 dB. Ensuring ahigh polarization extinction ratio throughout a series of moduleschallenges despite the use of single mode polarization maintainingfiber. Degradation of the PER can occur at fiber ferrule, fiber holder,or fusion splice in the series of modules.

Levels of PER above 23 dB may be obtained using linear-polarizingoptical components in the modules, such as shown in FIG. 11A wherein thepolarization beamsplitter 722 operates as a polarization filter therebyproviding a substantially linear polarization. The rotatable waveplate720 adjusts the polarization output from the fiber 716 preferably toreduce the amount of light that is filtered out and lost by thepolarization beamsplitter 722. In other embodiments, the optical fiber716 may be rotated to alter the polarization instead of or in additionto adjustment of the rotatable waveplate 720.

Use of linear-polarizing components in the modules that containpolarization degrading elements such as fiber ferrule, fiber holder, orfusion splice is advantageous. The linear polarizers counter thesuperposition of the phase shift from each polarization degradingelement. A superposed phase shift of 10 degrees may reduce the PER toabout 15 dB in which case intensity fluctuation through a linearpolarizer might be more than about 4%. In contrast, by embedding linearpolarizers throughout the series of modules, the PER of the aggregatesystem can be substantially controlled such that the intensityfluctuation is below about 1%, provided that the PER of the individualmodule and splice is above about 20 dB.

Preferably, the optical elements such as the first lenses 718, therotatable waveplate 720, the MacNeille polarizer 722, and the saturableabsorber 724 comprise micro-optics or are sufficiently small to providefor a compact module. The elements in the housing 712 are alsopreferably securely fastened to a base of the housing such as by laserwelding. The housing 712 may be sealed and thermally insulated as well.In various preferred embodiments, these modules conform to Telcordiastandards and specifications.

FIG. 11B illustrates one embodiment of a variable attenuator module 730comprising a housing 732 that contains optical components for providinga controllable amount of optical attenuation. A first optical fiberconnector 734 comprising an optical fiber 736 having an angle polishedor cleaved endface passes through one sidewall of the housing 732 intoan inner region of the housing containing the plurality of opticalcomponents. These optical components include a first lens 738 forcollecting and preferably collimating light output from the opticalfiber 736, a variable waveplate 740 and a polarization selective opticalelement 742. A second optical fiber connector 744 comprising an opticalfiber 745 having an angle polished or cleaved endface passes throughanother sidewall of the housing 732 into the inner region containing theoptical components. The variable waveplate 740 comprises a rotatablewaveplate mounted on a rotatable wheel 746 and the polarizationselective optical element 742 comprises a polarization beamsplitter suchas a MacNeille prism. A second lens 748 disposed between thepolarization selective optical element 742 couples light between thepolarization beamsplitter 742 and the second optical fiber 745. Anoptical path is formed from the first optical fiber 736 through thewaveplate 740 and prism 742 to the second optical fiber connector 744.

The waveplate 740 can be rotated to vary the distribution of light intoorthogonal polarizations. The polarization beamsplitter 742 can be usedto direct a portion of the light out of the optical path between thefirst and second fiber connectors 734, 744, depending on the state ofthe waveplate 740. Accordingly, a user, by rotating the waveplate 740and altering the polarization of light can control the amount of lightcoupled between the first and second optical fiber connectors 734, 744and thereby adjust the level of attenuation.

Preferably, the optical elements such as the first and second lenses738, 748, the rotatable waveplate 740 and the MacNeille polarizer 742comprise micro-optics or are sufficiently small to provide for a compactmodule. The elements in the housing 732 may be laser welded or otherwisesecurely fastened to a base of the housing. The housing 732 may besealed and thermally insulated as well. In various preferredembodiments, these modules conform to Telcordia standards andspecifications.

FIG. 11C illustrates one embodiment of an isolator module 760 comprisinga housing 762 that contains optical components for providing a opticalisolation. A first optical fiber connector 764 comprising an opticalfiber 766 having an angle polished or cleaved endface passes through onesidewall of the housing 762 into an inner region of the housingcontaining the plurality of optical components. These optical componentsinclude a first lens 768 for collecting and preferably collimating lightoutput from the optical fiber 766, an optical isolator 770, and abeamsplitter 772. A second optical fiber connector 774 comprising anoptical fiber 775 having an angle polished or cleaved endface passesthrough another sidewall of the housing 762 into the inner regioncontaining the optical components. The isolator 770 may comprise forexample a Faraday rotator and linear polarizers (not shown). Thebeamsplitter 772 may comprise a plate or wedge that directs a portion ofthe beam to a third fiber connector 780. In other embodiments, a lensmay couple light between the beamsplitter 772 and this third fiberconnector 780. The third fiber connector 780 may comprise a tap fortapping off a portion of the light propagating between the first opticalfiber 766 and the second optical fiber 775 and is not generally involvedin the operation of the isolator 770. Accordingly, the beamsplitter 772and tap 780 may be excluded from other embodiments of the isolatormodule design. The tap 780, however, may be useful for providingfeedback for laser systems as described elsewhere herein. A second lens778 disposed between the beamsplitter 772 and the second optical fiber775 couples light between the beamsplitter 772 and the second opticalfiber. An optical path is formed from the first optical fiber 766through the isolator 770 and beamsplitter 772 to the second opticalfiber connector 774. This optical path, however, is substantiallyuni-directional as a result of the isolator 770.

This module 760 may further comprise a waveplate and a polarizationselective optical element to assist in maintaining polarization asdescribed in connection with the saturable absorber module 710 shown inFIG. 11A. In other embodiments, e.g., containing a polarizationselective element such as a linear polarizer, the optical fibers may berotated to alter the polarization instead of or in addition to providinga rotatable waveplate.

Preferably, the optical elements such as the first and second lenses768, 778, the isolator 770 and the prism 772 comprise micro-optics orare sufficiently small to provide for a compact module. The elements inthe housing 762 may be laser welded or otherwise securely fastened to abase of the housing. The housing 762 may be sealed and thermallyinsulated as well. In various preferred embodiments, these modulesconform to Telcordia standards and specifications.

Other designs may be employed that differ from the design configurationsdepicted in connection with the modules 710, 730, 760 shown in FIGS.11A-11C. For example, other components may be added, alternativecomponents may be used, or the arrangement and configuration of thecomponents in the modules may be different. In some cases, componentsmay be removed. The housing may also be different. Still othervariations are possible.

FIG. 11D depicts an exemplary schematic representation of an oscillatorsuch as described above in reference to FIGS. 2A and 2B. In oneembodiment, an oscillator 600 comprises a saturable absorber module 602optically coupled to a cavity fiber assembly module 604. The two modulescan be optically coupled by a fiber pigtail, and the length of the fiberbetween the modules 602, 604 can be varied to contribute to a desiredgroup delay dispersion in the cavity.

The saturable absorber module 602 and/or the cavity fiber assemblymodule 604 may be coupled to respective temperature control components606 and 608. In one embodiment, the temperature control components 606and 608 comprise Peltier elements that provide a temperature controlover a relatively large range of temperatures. The temperaturecontrollers can be used, for example, to adjust the temperature of thegain fiber in the oscillator to stabilize gain and reduce noise.

FIG. 12 illustrates one approach to optically coupling the variousmodular components. An exemplary coupling 610 shows a first pigtailfiber 612 from a first component 614 coupled to a second pigtail fiber616 from a second component 618 by a splice 640. In one implementation,the splices are fusion spliced. As is known, such a splice providesadvantageous features associated with optical fibers as the two fibersegments are merged into one physically connected fiber optic path. Incomparison with bulk optics, complicated and potentially fragilealignment and positioning are not required once the fibers are coupledtogether. The splice thereby provides a substantially consistenttransmission of signals between the coupled components. Such techniquescan be readily implemented and thus improve manufacturability and reducecost. Using such techniques, the fibers' positions and coupling can alsobe made less vulnerable to environment changes thereby yielding animproved stability of the laser system. Other techniques such as buttcoupling can also be employed in other embodiments.

Optical fibers are also compact and lightweight in comparison, forexample, to lens systems, although lenses and other bulk optics may beused, for example, in different modules. Components coupled in theforegoing manner can be arranged in a variety of ways. Because theoptical interconnection between modular components is provided by afiber, the modules may not need to be aligned optically and the modulescan be arranged and packaged in a flexible manner.

FIG. 13 illustrates an exemplary arrangement of modules where thealignment of the axes through the module (i.e., a direction that may beassociated with freespace input and output coupling) is not arestrictive design limitation. As shown in FIG. 13, an exemplarycoupling 620 optically couples a first component 622 to a secondcomponent 624 via a splice 626. Accordingly, the second component 624 isoriented such that the axis through the first component 622 need not bealigned with the axis through of the second component. Similarly, bulkoptics such as reflectors or mirrors are not needed to provide opticalconnection between the two components 622, 624.

As described above, the modules may comprise one or more opticalelements supported within a housing. These optical elements may be bulkoptics such as lens and mirrors or other physical optics or may comprisewaveguide structures such as planar waveguides. In some embodiments,fiber optic components may be included in such housings although opticalfibers may be connected to the fiber pigtails extending from thehousings and may not have individual housings. For example, one lasersystem may comprise a saturable absorber in a housing having an opticalpigtail extending therefrom (such as in FIG. 11A) that is spliced to anoscillator fiber at least a portion of which is doped to provide gainand that includes a fiber Bragg grating as a partial reflector. Thefiber Bragg grating may be spliced to an input pigtail of an isolator(such as in FIG. 11C) comprising bulk optics components disposed withina separate housing. An output pigtail extending from the isolator modulemay be spliced to another modulate containing bulk optics that form avariable attenuator (such as in FIG. 11B). The variable attenuatormodule may have an output fiber pigtail optical connected to a gainfiber comprising a fiber amplifier. The fiber amplifier may be splicedto a fiber Bragg grating or photonic crystal fiber that providescompression. Other configurations are also possible.

The modular approach described above offers many advantages. Thefiber-based modular approach aids in designing, addressing limitations,and providing practical solutions for applications in medicine,industry, and other environments. In many cases, design of reliable andcomplex optical and laser systems is generally a difficult taskinvolving expenditure of excessive resources and extensive amounts oftime. Using modular opto-mechanical elements, an ultrashort pulse lasercan be more efficiently designed for particular applications.Advantageously, the design of the system may be first simplified, whichcan be accomplished by dividing the system into several functionalgroups. The functionalities can be achieved with different modules whichcan be separately designed and tested. Design assessment of reliabilitycan be achieved at much lower cost involving less time and lessresources. Engineering of separate modules for product development ismore manageable.

The modular approach can also significantly simplify the assemblyprocess and improve the manufacturability of the laser systems. Thelaser systems can be assembled with simple fusion splices withoutfree-space alignment. Such process can decrease labor costs and increasethe operation stability, reliability, and repeatability. Repair,replacement, and upgrading may also be facilitated by the modularapproach as the modules may be replaceable and/or interchangeable.

In various preferred embodiments, the optics within the modules comprisemicro-optics elements although other types of optics are possible. Useof micro-optics and fiber optics provides compactness. Preferably, theoptics are secured to the housing and the housing provides sufficientprotection such that the laser systems are rugged and robust. In variousembodiments, the housings may comprise thermal insulation and/or may behermetically sealed to reduce build-up of condensation, moisture, dust,dirt, or other contamination that may interfere with the operation orreliability of the optical elements.

The modular design disclosed herein provides other advantages in thedesign and performance of high power short pulse laser systems. Reducedform factor and mass of the components may enable a high-degree ofoptical stability. Environmental stability of the system can be improvedalso by controlling the temperature of the devices. In some embodiments,the laser system can be packaged to meet the telecommunication standardsin performance and quality. In some embodiments, for example, the lasersystem or portions thereof can be packaged in compliance with aTelcordia reliability assurance requirements such as GR-1221-CORE andGR-468-CORE.

The recent unprecedented growth of the telecom industry has resulted inthe development of a mature fiber technology and reliable andcost-effective components. However, due to the nature oftelecommunication requirements, the commercial fiber components aremostly limited by low power handling capability and continuous-wave (CW)operation. High average power (>200 mW) and ultrafast pulse operationinvolved specially designed components. Preferably, however, anultrashort fiber laser and amplifier system may be provided that is incompliance of the applicable Telcordia reliability assurancerequirements, for example, GR-468-CORE and GR-1221-CORE.

Environmentally stable laser design is highly desirable for industrialapplication. A preferred industrial laser system can, for example, becharacterized by an output power variation below 0.5 dB over anenvironmental temperature range from 0 to 50 degree Celsius and bycompliance of vibration, thermal shock, high temperature storage andthermal cycling test criteria in Telcordia GR-468-CORE and GR-1221-CORE.This target can be achieved by functional segmentation of components andusing appropriate packaging in the modules such as for exampleTelcordia-qualified packaging technology. Accordingly, preferably, themodules are designed and manufactured to comply with telecom standardsand quality.

As described above, various embodiments comprise a high power ultrashortpulses laser system having an output power over about 200 mW and a pulsewidth less than about 200 femtoseconds. Certain embodiments may employ ashort length of gain fiber to enhance the gain stability of theoscillator against environmental temperature variation. In someembodiments, the cavity dispersion may be managed by adding undopedpolarization maintaining fiber, which may be provided by a fiber pigtailintegrated with the saturable absorber module. As discussed above, thesaturable absorber and the optics associated with the saturable absorbermay be packaged with telecommunication packaging technology to form amodular toolkit in the oscillator system. In some designs, the modulecan be integrated with a temperature controller.

In certain preferred embodiments, the light from the oscillator may beamplified. Also, the quality of the amplified pulse, such as minimumpedestal and compressibility, may be controlled by manipulating thespectral detail of the seed pulse out of oscillator. Such manipulationmay be accomplished by using a spectral filter although other designsare possible. Depending on the detail of the chirp of the pulse andnonlinear phase distortion in the amplifier, in some embodiments, agrating pair having a properly selected center wavelength and bandwidthmay provide suitable balance for the chirp such that the pulse can becompressed with a sufficiently high pulse quality. Furthermore, thespectral property of the filter can be further tailored for pedestalflattening in some embodiments. A specific pedestal flattening filter,for example, can be used. In certain embodiments, a parabolic amplifiermay be used to amplify the seed pulse. In case of seed spectralbandwidth larger than 10 nm, a filter-type element may be inserted infront of amplifier to shorten the seed pulse width to improve oroptimize the amplification and compressibility of the amplified pulse.

Other embodiments having different designs and configurations arepossible and should not be limited to those described above. Forexample, although the various systems disclosed herein can operate inthe wavelength of around 1050 nm, the concepts of the present teachingscan also be applied to laser systems operating at other wavelengths.

Moreover, the above description of the preferred embodiments has beengiven by way of example. From the disclosure given, those skilled in theart will not only understand the present invention and its attendantadvantages, but will also find apparent various changes andmodifications to the structures and methods disclosed. It is sought,therefore, to cover all such changes and modifications as fall withinthe spirit and scope of the invention, as defined by the appendedclaims, and equivalents thereof.

FIG. 14 represents an exemplary embodiment of the present inventionembodied in a fiber laser cavity 1100. A polarization-maintaining gainfiber 1101 has a core 1102 and cladding region 1103. The fiber core 1102is doped with rare-earth ions, such as Yb, Nd, Er, Er/Yb, Tm or Pr, toproduce gain at a signal wavelength when the laser is pumped with diodelaser 1104. The fiber core can be single-mode or multi-mode. The fiberlaser cavity 1100 further contains an integrated fiber polarizer 1105and a chirped fiber Bragg grating 1106. Both of these elements, 1105 and1106, are generally constructed of short fiber pigtails (e.g., 0.001-1 min length), which are preferably fusion-spliced to fiber 1101 usingsplices 1107, 1108 and 1109. Alternatively, fiber polarizer 1105 can bespliced in front of beam expander 1110. When using multi-mode fiber,splice 1107 is selected to match the fundamental mode in the gain fiber1101.

An exemplary integrated fiber polarizer in accordance with the inventioncomprises a polarization-maintaining undoped polarizer fiber (PF), withtwo orthogonal polarization axes, where the loss along one polarizationaxis is significantly higher than the loss along the other polarizationaxis. Alternatively, a very short section (less than 1 cm) ofnon-birefringent fiber (i.e., non-polarization-maintaining fiber) can besandwiched between two sections of polarization-maintaining fiber, wherethe polarization axes of the polarization-maintaining fibers are alignedwith respect to each other. By side-polishing the non-birefringentfiber, e.g., down to the evanescent field of the fiber core, along oneof the axes of the birefringent fiber, and coating the polished regionwith metal, high extinction polarization action can be obtained alongone of the axes of the birefringent fiber. The design of side-polishedfiber polarizers is well known in the field and not discussed furtherhere.

For optimum laser operation, the fiber polarization axes of the PF arealigned parallel to the polarization axes of the gain fiber 1101. Toensure stable modelocked operation, the polarizer preferably effectivelyeliminates satellite pulses generated by any misalignment between thepolarization axes of the PF and the gain fiber 1101.

Neglecting any depolarization in the all-fiber polarizer itself, it canbe shown by applying a Jones matrix calculation method that for amisalignment of the polarization axes of gain fiber 1101 and fiberpolarizer 1105 by α degrees, the linear reflectivity R from theright-hand side of the cavity varies approximately between R=1-0.5 sin²2α and R=1 depending on the linear phase in the gain fiber 1101. If thegroup delay along the two polarization axes of the gain fiber is largerthan the intra-cavity pulse width, any satellite pulse is suppressed bysin⁴α after transmission through the polarizer. Typical fiber splicingmachines can align polarization-maintaining fibers with an angularaccuracy of less than ±2°; hence any reflectivity variation due todrifts in the linear phase between the two polarization eigenmodes offiber 1101 can be kept down to less than 3×10⁻³, whereas (forsufficiently long fibers) any satellite pulses obtained aftertransmission through the polarizer can be kept down to less than 6×10⁻⁶when using an integrated polarizer.

The chirped fiber Bragg grating 1106 is preferably spliced to the PF1105 at splice position 1108 and written in non-polarization-maintainingfiber. In order to avoid depolarization in the fiber Bragg grating, theBragg grating pig-tails are preferably kept very short, e.g., a lengthsmaller than 2.5 cm is preferable between splice locations 1108 and1109. To obtain a linear polarization output, a polarization-maintainingfiber pig-tail is spliced to the left-side of the fiber Bragg grating atsplice location 1109. The laser output is obtained at a first fiber (orcavity) end 1111, which is preferably angle-cleaved to avoidback-reflections into the cavity.

Fiber Bragg grating 1106 serves two functions. First, it is used as anoutput mirror (i.e., it feeds part of the signal back to the cavity)and, second, it controls the amount of cavity dispersion. In the presentimplementation, the chirped fiber Bragg grating has a negative(soliton-supporting) dispersion at the emission wavelength in thewavelength region near 1060 nm and it counter-balances the positivematerial dispersion of the intra-cavity fiber. To produce the shortestpossible pulses (with an optical bandwidth comparable to or larger thanthe bandwidth of the gain medium), the absolute value of the gratingdispersion is selected to be within the range of 0.5-10 times theabsolute value of the intra-cavity fiber dispersion. Moreover, the fiberBragg grating is apodized in order to minimize any ripple in thereflection spectrum of the grating. Accordingly, the oscillation ofchirped pulses is enabled in the cavity, minimizing the nonlinearity ofthe cavity and maximizing the pulse energy. Chirped pulses arecharacterized in having a pulse width which is longer than the pulsewidth that corresponds to the bandwidth limit of the corresponding pulsespectrum. For example the pulse width can be 50%, 100%, 200% or morethan 1000% longer than the bandwidth limit.

Alternatively, the oscillation of chirped pulses is also enabled byusing negative dispersion fiber in conjunction with positive dispersionchirped fiber Bragg gratings. Pulses with optical bandwidth comparableto the bandwidth of the gain medium can also be obtained with thisalternative design.

A SAM 1112 at a second distal fiber end 1113 completes the cavity. In anexemplary implementation a thermally expanded core (TEC) 1110 isimplemented at cavity end 1113 to optimize the modelocking performanceand to allow close coupling of the SAM 1112 to the second fiber end 1113with large longitudinal alignment tolerances. Etalon formation betweenthe fiber end 1113 and the SAM 1112 is prevented by an anti-reflectioncoating deposited on fiber end 1113 (not separately shown). In thevicinity of the second fiber end 1113, fiber 1101 is further insertedinto ferrule 1114 and brought into close contact with SAM 1112. Fiber1101 is subsequently fixed to ferrule 1114 using, for example, epoxy andthe ferrule itself is also glued to the SAM 1112.

The pump laser 1104 is coupled into the gain fiber 1101 via a lenssystem comprising, for example, two lenses 1115 and 1116 and a V-groove1117 cut into fiber 1101. Such side-coupling arrangements are describedin, for example, U.S. Pat. No. 5,854,865 ('865) to L. Goldberg et al.Alternatively, fiber couplers can be used for pump light coupling.

An exemplary design for a SAM in accordance with the present inventionis shown in FIG. 15 a. For example, SAM 1200 includes an InGaAsP layer1201 with a thickness of 50-2000 nm. Further, layer 1201 is grown with abandedge in the 1 μm wavelength region; the exact wavelength is definedby the sought emission wavelength of the fiber laser and can varybetween 1.0-1.6 μm. The InGaAsP layer 1201 is further coated orprocessed with a reflective material such as Au or Ag. A dielectricmirror or semiconductor Bragg reflector 1202 is located beneath layer1201 and the entire structure is attached to heat sink 1203, based on,for example, metal, diamond or sapphire.

In order to cover a broad spectral range (e.g., greater than 100 nm)metallic mirrors are preferred. When using a metallic mirror it isadvantageous to remove the substrate (InP) by means of etching. Whenusing HCl as an etching solvent the etching selectivity between InGaAsPand InP can be low, depending on the compound composition of InGaAsP. Anetch-stop layer is beneficial between the substrate and the InGaAsPlayer. InGaAs can be a proper etch-stop layer. When adding an InGaAslayer with a band-gap wavelength shorter than 1.03 μm, latticerelaxations can be avoided by keeping the thickness below 10 nm.

The InGaAsP layer can further be anti-reflection coated with layer 1204on its upper surface to optimize the performance of the SAM. Because ofthe saturable absorption by InGaAsP, the reflectivity of the SAMincreases as a function of light intensity, which in turn favors thegrowth of short pulses inside the laser cavity. The absence of Al in thesaturable absorber layer prevents oxidization of the semiconductorsurfaces in ambient air and thus maximizes the life-time and powerhandling capability of the structure.

Instead of InGaAsP, any other Al-free saturable semiconductor can alsobe used in the construction of the SAM. Alternatively, Al-containingsemiconductors can be used in the SAM with appropriately passivatedsurface areas. Surface passivation can, for example, be accomplished bysulfidization of the semiconductor surface, encapsulating it with anappropriate dielectric or with an Al-free semiconductor cap layer. AnAlGaInAs absorber layer grown lattice-matched on InP can besurface-passivated with a thin (about 10 nm range) cap layer of InP.AlGaInAs with a higher bandgap energy than the absorber layer can alsobe used for a semiconductor Bragg reflector in combination with InP.Among concepts for semiconductor Bragg mirrors lattice-matched to InP,an AlGaInAs/InP combination has an advantage over an InGaAsP/InP Braggreflector due to its high refractive index contrast.

Instead of a bulk semiconductor saturable absorber, a MQW saturableabsorber structure as shown in FIG. 15 b may also be used. In this case,the SAM 1205 conveniently comprises MQW structures 1206, 1207 and 1208separated by passive spacer layers 1209-1212 in order to increase thesaturation fluence and depth-selective ion-implantation concentration ofeach MQW section. Additional MQW structures can further be used,similarly separated by additional passive spacer layers. To reduce thewavelength and location sensitivity of the MQW saturable absorbers, thewidth of the spacer layers varies from spacer layer to spacer layer.Furthermore, multiple bulk layers with thicknesses larger than 500 Å canreplace the MQW structure. The MQW layers, in turn, can contain severallayers of quantum wells and barriers such as, for example, InGaAs andGaAs, respectively. Top surface 1209 can further be anti-reflectioncoated (not shown); a reflective structure is obtained by includingmirror structure 1213. The entire structure can be mounted on heat sink1214.

The control of the response time of the saturable absorption forconcomitant existence of fast and slow time constants is realized byintroducing carrier trap centers with depth controlled H+(or other ions)implantation. The implantation energy and dose are adjusted such thatpart of the absorbing semiconductor film contains a minimal number oftrap centers. For example the semiconductor layer with the minimalnumber of trap centers can be selected to be at the edge of the opticalpenetration range of exciting laser radiation. Such a design serves onlyas an example and alternatively any semiconductor area within theoptical penetration range can be selected to contain a minimal number oftrap centers. Hence distinctive bi-temporal carrier relaxation isobtained in the presence of optical excitation. As an illustration ofdepth selective ion implantation, FIG. 16 shows the measurement of thedepth profile of H+ ion implantation of an InGaAsP absorber film takenfrom secondary ion mass spectroscopy (SIMS).

The obtained bi-temporal carrier life-time obtained with thesemiconductor film with a proton concentration as shown in FIG. 16, isfurther illustrated in FIG. 17. Here the reflectivity modulation (dR/R0)of a semiconductor saturable mirror due to excitation of the saturablemirror with a high energy short pulse at time t=0 is shown as a functionof time delay. The measurement was obtained with a pump-probe technique,as well known in the art. FIG. 17 clearly displays the bi-temporalresponse time due to fast (<1 ps) and slow (>>100 ps) recovery. Thedistinctive fast response originates from the depth region with hightrap concentration, while the slow component results from the rear depthregion with a much lower trap center concentration.

When employing this absorber in the laser system described with respectto FIG. 14, Q-switched mode-locking is obtained at intracavity powerlevels of a few mW. At the operating pump power level, stable cwmode-locking evolving from Q-switch mode-locking is observed. Incontrast, no Q-switching and no mode-locking operation is obtained withthe same semiconductor material implanted uniformly with protons withoutbi-temporal carrier relaxation (exhibiting only fast carrierrelaxation).

We emphasize that the description for FIG. 16 and FIG. 17 is to serve asan example in controlling 1) the fast time constant, 2) the slow timeconstant, 3) the ratio of the fast and slow time constants, 4) theamplitude of the fast response, 5) the amplitude of the slow response,and finally 6) the combination of all of the above by ion implantationin a saturable absorber. Thus, the concept depicted hereby can beapplicable for any type of laser modelocked with a saturable absorber.Specifically, in the presence of un-avoidable large spuriousintra-cavity reflections such as in fiber lasers or thin disk lasers (F.Brunner et al., Sub-50 fs pulses with 24 W average power from apassively modelocked thin disk Yb:YAG laser with nonlinear fibercompression, Conf. on Advanced Solid State Photonics, ASSP, 2003, paperNo.: TuA1), the disclosed engineerable bi-temporal saturable absorberscan greatly simplify and stabilize short pulse formation.

The preferred implantation parameters for H+ ions in GaAs or InP relatedmaterials including MQW absorbers are as follows: The doses and theimplantation energies can be selected from 10¹² cm⁻² to 10¹⁷ cm⁻² andfrom 5 keV to 200 keV, respectively, for an optically absorbing layerthickness between 50 nm and 2000 nm. For MQW absorbers, the selectiveion-implantation depth is rather difficult to measure because theshallow MQW falls into the implantation peak in FIG. 16. However, withthe separation of MQW sections with spacers 1209-1212 (as shown in FIG.15 b) it is feasible to employ depth selective ion implantation. Forarsenic implantation, the implantation parameters for 50-2000 nmabsorbing layer spans from 10¹² cm⁻² to 10¹⁷ cm⁻² for the dosage and animplantation energy range of 100 keV to 1000 keV. In case of MQWsaturable absorbers, the implantation range is preferably selectedwithin the total thickness of the semiconductor layer structurecontaining MQW sections and spacers. In addition to H⁺ and arsenic, anyother ions such as for example Be can be implanted with controlledpenetration depth by adjusting the above recipes according to thestability requirements of the desired laser. Depth selective ionimplantation is illustrated in FIG. 15 a in which dashed curve 1201 arepresents the H⁺ ion depth profile of FIG. 16.

FIG. 18 a illustrates an alternative implementation of the fiber end andSAM coupling in FIG. 14. Here cavity 1300 comprises an angle-polishedthermal-diffusion expanded core (TEC) 1301. Fiber end 1302 is broughtinto close contact with SAM 1303 and fiber 1304 is rotated insideferrule 1305 to maximize the back reflection from SAM 1303. Ferrule 1305is further angle-polished and SAM 1303 is attached to the angle-polishedsurface of ferrule 1305. As shown in FIG. 18 a, fiber 1304 isconveniently glued to the left-hand side of ferrule 1305. A wedge-shapedarea between the fiber surface 1302 and SAM 1303 greatly reduces thefinesse of the etalon between the two surfaces, which is required foroptimum modelocked laser operation.

Instead of TEC cores, more conventional lenses or graded index lensescan be incorporated between the fiber end and the SAM to optimize thebeam diameter on the SAM. Generally, two lenses are required. A firstlens collimates the beam emerging from the fiber end, and a second lensfocuses the beam onto the SAM. According to present technology, evenconventional lenses allow the construction of a very compact package forthe second fiber end. An implementation with two separate collimationand focusing lenses is not separately shown. To minimize unwantedbackreflections into the fiber cavity and to minimize the number ofcomponents, a single lens can be directly fused to the fiber end asdepicted in FIG. 18 b. As shown in FIG. 18 b, assembly 1306 contains SAM1303 and fiber 1304 as well as lens 1307, which focuses the optical beamonto the SAM. Lens 1307 can also include a graded index lens.

To minimize aberrations in assembly 1306, an additional lens can also beincorporated between lens 1307 and SAM 1303. Such an assembly is notseparately shown. Alternatively, a lens can be directly polished ontofiber 1304; however, such an arrangement has the disadvantage that itonly allows a beam size on the SAM which is smaller than the beam sizeinside the optical fiber, thereby somewhat restricting the designparameters of the laser. To circumvent this problem, a lens surface canbe directly polished onto the surface of a TEC; such an implementationis not separately shown. Another alternative is to exploit agraded-index lens design attached directly onto the fiber tip to varythe beam size on the SAM. In the presence of air-gaps inside theoscillator a bandpass filter 1308 can be incorporated into the cavity,allowing for wavelength tuning by angular rotation as shown, forexample, in FIG. 18 b.

Passive modelocking of laser cavity 1100 (FIG. 14) is obtained when thepump power exceeds a certain threshold power. In a specific, exemplary,implementation, polarization-maintaining fiber 1101 was doped with Ybwith a doping level of 2 weight %; the doped fiber had a length of 1.0m; the core diameter was 8 um and the cladding diameter was 125 um. Anadditional 1.0 m length of undoped polarization-maintaining fiber wasalso present in the cavity. The overall (summed) dispersion of the twointra-cavity fibers was approximately +0.09 ps². In contrast, the fibergrating 106 had a dispersion of −0.5 ps², a spectral bandwidth of 10 nmand a reflectivity of 50%. The grating was manufactured with a phasemask with a chirp rate of 80 nm/cm.

When pumping with an optical power of 1.0 W at a wavelength of 910 nm,the laser produced short chirped optical pulses with a full width halfmaximum width of 1.5 ps at a repetition rate of 50 MHz. The averageoutput power was as high as 10 mW. The pulse bandwidth was around 2 nmand hence the pulses were more than two times longer than thebandwidth-limit which corresponds to around 800 fs.

Alternatively, a fiber grating 1106 with a dispersion of −0.1 ps²,closely matching the dispersion of the intra-cavity fiber, wasimplemented. The fiber grating had a reflectivity of 9% and a spectralbandwidth of 22 nm centered at 1050 nm. The grating was manufacturedwith a phase mask with a chirp rate of 320 nm/cm. The laser thenproduced chirped optical pulses with a full-width half maximum width of1.0 ps at a repetition rate of 50 MHz with an average power of 25 mW.The pulse spectral bandwidth was around 20 nm and thus the pulses werearound 10 times longer than the bandwidth limit, which corresponds toaround 100 fs. The generation of pulses with a pulse width correspondingto the bandwidth limit was enabled by the insertion of a pulsecompressing element; such elements are well known in the state of theart and are not further discussed here. The generation of even shorterpulses can be generated with fiber gratings with a bandwidth of 40 nm(and more) corresponding to (or exceeding) the spectral gain bandwidthof Yb fibers.

Shorter pulses or pulses with a larger bandwidth can be convenientlyobtained by coupling the fiber output into another length of nonlinearfiber as shown in FIG. 19. Here, assembly 1400 contains the integratedfiber laser 1401 with pig-tail 1402. Pig-tail 1402 is spliced (orconnected) to the nonlinear fiber 1403 via fiber splice (or connector)1404. Any type of nonlinear fiber can be implemented. Moreover, fiber1403 can also comprise a fiber amplifier to further increase the overalloutput power.

In addition to cladding pumped fiber lasers, core-pumped fiber laserscan be constructed in an integrated fashion. Such an assembly is shownin FIG. 20. The construction of cavity 1500 is very similar to thecavity shown in FIG. 14. Cavity 1500 contains polarization-maintainingfiber 1501 and integrated fiber polarizer 1502. Fiber 1501 is preferablysingle-clad, though double-clad fiber can also be implemented. Thechirped fiber grating 1503 again controls the dispersion inside thecavity and is also used as the output coupler. Fiber 1501, fiberpolarizer 1502, fiber grating 1503 and the polarization-maintainingoutput fiber are connected via splices 1504-1506. The output from thecavity is extracted at angle-cleaved fiber end 1507. SAM 1508 containsanti-reflection coated fiber end 1509, located at the output of the TEC1510. Fiber 1501 and SAM 1508 are fixed to each other using ferrule1511. The fiber laser is pumped with pump laser 1512, which is injectedinto the fiber via wavelength-division multiplexing coupler 1513.

In addition to chirped fiber gratings, unchirped fiber gratings can alsobe used as output couplers. Such cavity designs are particularlyinteresting for the construction of compact Er fiber lasers. Cavitydesigns as discussed with respect to FIGS. 14 and 20 can be implementedand are not separately shown. In the presence of fiber gratings as shownin FIGS. 14 and 20, the fiber gratings can also be used as wavelengthtuning elements. In this, the fiber gratings can be heated, compressedor stretched to change their resonance condition, leading to a change incenter wavelength of the laser output. Techniques for heating,compressing and stretching the fiber gratings are well known.Accordingly, separate cavity implementations for wavelength tuning via amanipulation of the fiber grating resonance wavelength are notseparately shown.

In the absence of a fiber grating, a mirror can be deposited or attachedto one end of the fiber cavity. The corresponding cavity design 1600 isshown in FIG. 21. Here, it is assumed that the fiber 1601 is corepumped. The cavity comprises an intra-cavity all-fiber polarizer 1602spliced to fiber 1601 via splice 1603. Another splice 1604 is used tocouple WDM 1605 to polarizer 1602. Polarization maintaining WDM 1605 isconnected to pump laser 1606, which is used to pump the fiber laserassembly. Saturable absorber mirror assembly 1607, as describedpreviously with respect to FIGS. 14 and 20, terminates one cavity endand is also used as the passive modelocking element.

A second fiber polarizer 1608 is spliced between WDM 1605 andpolarization-maintaining output coupler 1609 to minimize the formationof satellite pulses, which can occur when splicing sections ofpolarization maintaining fiber together without perfect alignment oftheir respective polarization axes, as discussed in U.S. patentapplication Ser. No. 09/809,248. Typically, coupler 1609 has a couplingratio of 90/10 to 50/50, i.e., coupler 1609 couples about 90-50% of theintra-cavity signal out to fiber pig-tail 1610. Pig-tail 1610 can bespliced to a fiber isolator or additional fiber amplifiers to increasethe pulse power. The second cavity end is terminated by mirror 1611.Mirror 1611 can be directly coated onto the fiber end face or,alternatively, mirror 1611 can be butt-coupled to the adjacent fiberend.

The increase in stability of cavity 1600 compared to a cavity where theoutput coupler fiber, the WDM fiber and gain fiber 1601 are directlyconcatenated without intra-fiber polarizing stages, can be calculatedusing a Jones matrix formalism even when coherent interaction betweenthe polarization axes of each fiber section occurs.

Briefly, due to the environmental sensitivity of the phase delay betweenthe polarization eigenmodes of each fiber section, for N directlyconcatenated polarization-maintaining fibers the reflectivity of a fiberFabry-Perot cavity can vary between R=1 and R=1−(N×α)², where α is theangular misalignment between each fiber section. Further, it is assumedthat α is small (i.e., α<<10°) and identical between each pair of fibersections. Also, any cavity losses are neglected. In fact, it isadvantageous to analyze the possible leakage L into the unwantedpolarization state at the output of the fiber cavity. L is simply givenby L=1−R. For the case of N concatenated fiber sections, the maximumleakage is thus (N×α)².

In contrast, a cavity containing N−1 polarizers in-between N sections ofpolarization-maintaining fiber is more stable, and the maximum leakageis L=2×(N−1)α². Here, any depolarization in the fiber polarizers itselfis neglected. For instance, in a case where N=3, as in cavity 1600, theleakage L into the wrong polarization axis is 2×(3−1)/3³=4/9 timessmaller compared to a cavity with three directly concatenated fibersections. This increase in stability is very important in manufacturingyield as well as in more reproducible modelocked operation in general.

In constructing a stable laser, it is also important to consider theconstruction of WDM 1605 as well as output coupler 1609. Various vendorsoffer different implementations. An adequate optical representation ofsuch general polarization-maintaining fiber elements is shown in FIG.22. It is sufficient to assume that a general coupler 1700 comprises twopolarization-maintaining fiber sections (pig-tails) 1701, 1702 with acoupling point 1703 in the middle, where the two polarization axes ofthe fiber are approximately aligned with respect to each other.

In order to ensure pulse stability inside a passively modelocked laser,the group-velocity walk-off along the two polarization axes of fibersections 1701, 1702 should then be longer than the full-width halfmaximum (FWHM) pulse width of the pulses generated in the cavity. Forexample, assuming a birefringent fiber operating at a wavelength of 1550nm with a birefringence of 3×10⁻⁴ corresponding to a polarization beatlength of 5 mm at 1550 nm, the stable oscillation of soliton pulses witha FWHM width of 300 fs requires pig-tails with a length greater than 29cm. For 500 fs pulses, the pig-tail length should be increased to around50 cm.

Referring back to FIG. 21, if a fiber pig-tailed output is not required,mirror 1611 as well as output coupler 1609 can be omitted, and the 4%reflection from the fiber end adjacent to mirror 1611 can be used as aneffective output mirror. Such an implementation is not separately shown.

Alternatively, a fiber-pig-tail can be butt-coupled to mirror 1611 andalso be used as an output fiber pigtail. Such an implementation is shownin FIG. 23. Here, cavity 1800 comprises core-pumped fiber 1801, fiberpolarizer 1802 and SAM assembly 1803. The laser is pumped via WDM 1804connected to pump laser 1805. An appropriate mirror (or mirror coating)1806 is attached to one end of the cavity to reflect a part of theintra-cavity light back to the cavity and to also serve as an outputmirror element. Fiber pig-tail 1807 is butt-coupled to the fiber laseroutput mirror 1806 and an additional ferrule 1808 can be used tostabilize the whole assembly. The polarization axes of fiber 1807 and1801 can be aligned to provide a linearly polarized output polarization.Again, applying a Jones matrix analysis, cavity 1800 is more stable thancavity 1600, because it comprises only one intra-fiber polarizingsection. The maximum leakage in cavity 1800 compared to a cavitycomprising directly concatenated WDM and gain fiber sections is 50%smaller.

Similarly, a cladding pumped version of cavity 1600 can be constructed.Cavity 1900 shown in FIG. 24 displays such a cavity design. Fiber 1901is pumped via pump laser 1902, which is coupled to fiber 1901 via lensassembly 1903 and 1904 as well as V-groove 1905. Alternatively,polarization-maintaining multi-mode fiber couplers or star-couplerscould be used for pump power coupling. Such implementations are notseparately shown. One end of the laser cavity is terminated with SAMassembly 1906 (as discussed in regard to FIGS. 14, 20 and 21), which isalso used as the modelocking element. A single-polarization inside thelaser is selected via all-fiber polarizer 1907, which is spliced intothe cavity via splices 1908 and 1909. Polarization-maintaining outputcoupler 1910 is used for output coupling. The laser output is extractedvia fiber end 1911, which can further be spliced to additionalamplifiers. Cavity mirror 1912 terminates the second cavity end. Outputcoupler 1910 can further be omitted and the laser output can be obtainedvia a butt-coupled fiber pig-tail as explained with reference to FIG.23.

The cavity designs discussed with respect to FIGS. 14, 20, 21, 23 and 24follow general design principles as explained with reference to FIGS. 25a-25 c.

FIG. 25 a shows a representative modelocked Fabry-Perot fiber lasercavity 2000, producing a linear polarization state oscillating insidethe cavity containing one (or more) sections of non-polarizationmaintaining fiber 2001 and one (or more) sections of polarizationmaintaining fiber 2002, where the length of fiber section 2001 issufficiently short so as not to degrade the linear polarization stateinside the fiber laser cavity, more generally a predominantly linearpolarization state is oscillating everywhere within the intracavityfiber. The fiber laser output can be obtained from cavity end mirrors2003 or 2004 on either side of the cavity. To suppress the oscillationof one over the other linear polarization state inside the cavity,either fiber 2001 or 2002 has a polarization dependent loss at theemission wavelength.

FIG. 25 b shows a representative modelocked Fabry-Perot fiber lasercavity 2005, producing a linear polarization state oscillating insidethe cavity containing two (or more) sections of polarization maintainingfibers 2006, 2007, where the length of fiber sections 2006, 2007 issufficiently long so as to prevent coherent interaction of short opticalpulses oscillating inside the cavity and propagating along thebirefringent axes of fibers 2006, 2007. Specifically, for an oscillatingpulse with a FWHM width of τ, the group delay of the oscillating pulsesalong the two polarization axes of each fiber should be larger than τ.For oscillating chirped pulses τ represents the bandwidth-limited pulsewidth that corresponds to the oscillating pulse spectrum. Cavity 2005also contains end mirrors 2008 and 2009 and can further containsufficiently short sections of non-polarization maintaining fiber asdiscussed with reference to FIG. 25 a.

FIG. 25 c shows a representative modelocked Fabry-Perot fiber lasercavity 2010, producing a linear polarization state oscillating insidethe cavity containing one (or more) sections of polarization maintainingfiber 2011, 2012 and one (or more) sections of polarizing fiber (orall-fiber polarizer) 2013, where the length of fiber sections 2011, 2013is not sufficient to prevent coherent interaction of short opticalpulses oscillating inside the cavity and propagating along thebirefringent axes of fibers 2011, 2013, where the polarizing fiber issandwiched between the sections of short polarization maintaining fiber.Cavity 2010 further contains cavity end mirrors 2014 and 2015 and canfurther contain short sections of non-polarization maintaining fiber asdiscussed with reference to FIG. 25 a. Moreover, cavity 2010 (as well as2000 and 2005) can contain bulk optic elements 2016, 2017 (or any largernumber) randomly positioned inside the cavity to provide additionalpulse control such as wavelength tuning or dispersion compensation. Notethat the fibers discussed here can be single-clad, double-clad; thefibers can comprise also holey fibers or multi-mode fibers according tothe system requirement. For example polarization maintaining holeyfibers can be used for dispersion compensation, whereas multi-modefibers can be used for maximizing the output pulse energy. Cavitymirrors 2014, 2015, 2003, 2004 and 2008, 2009 can further comprise bulkmirrors, bulk gratings or fiber gratings, where the fiber gratings canbe written in short sections of non-polarization maintaining fiber thatis short enough so as not to perturb the linear polarization stateoscillating inside the cavity.

FIG. 26 serves as an example of a passively modelocked linearpolarization cavity containing holey fiber for dispersion compensation.Cavity 2100 contains fiber 2101, side-pumping assembly 2102 (directingthe pump light either into the cladding or the core of fiber 2101 asexplained before), saturable absorber mirror assembly 2103, all fiberpolarizer 2104 and fiber output coupler 2105 providing an output atfiber end 2106. All the above components were already discussed withrespect to FIG. 21. In addition, a length of polarization maintainingholey fiber 2106 is spliced to the cavity for dispersion compensationand the cavity is terminated on the left hand side by mirror 2107.

FIG. 27 serves as another example of a passively modelocked linearpolarization cavity containing a fiber grating for dispersioncompensation as applied to the generation of ultra-stable spectralcontinua. System 2400 comprises a small modification of the cavityexplained with respect to FIG. 20. System 2400 contains a fiber laser2401 generating pulses with a bandwidth comparable to the spectralbandwidth of the fiber gain medium 2402. Fiber laser 2401 furthercomprises saturable absorber mirror assembly 2403, wide bandwidth fibergrating 2404, polarization maintaining wavelength division multiplexing(WDM) coupler 2405, which is used to direct pump laser 2406 into fibergain medium 2402. Pump laser 2406 is preferably single-mode to generatethe least amount of noise.

To enable the oscillation of short pulses with a bandwidth comparable tothe bandwidth of the gain medium 2402, saturable absorber mirror 2403contains a bi-temporal saturable absorber, constructed with abi-temporal life-time comprising a 1st short life-time of <5 ps and a2nd long life-time of >50 ps. More preferable is a first life-time of <1ps, to allow pulse shaping of pulses as short as 100 fs and shorter. Byselecting the penetration depth of the implanted ions into the saturableabsorber, even tri-temporal saturable absorbers can be constructed.

The wide-bandwidth grating is preferably selected to approximately matchthe dispersion of the intra-cavity fibers. The wide-bandwidth gratingcan be made in short non-polarization maintaining fibers and it can bemade also in polarization maintaining fibers. In order to suppressdetrimental effects from cross coupling between the two polarizationaxes of the fiber grating, coupling to cladding modes in such largebandwidth fiber gratings should be suppressed. Gratings with suppressedcoupling to cladding modes can be made in optical fibers withphotosensitive core and cladding area, where the photosensitive claddingarea is index-matched to the rest of the cladding. Such fiber designsare well known in the state of the art and can for example bemanufactured with an appropriate selection of germania and fluorinedoping in the core and cladding regions and such fiber designs are notfurther discussed here. Because of the large generated bandwidth,splicing of such polarization maintaining gratings to the rest of thecavity without coherent coupling between the linear polarizationeigenmodes is no problem. Alternatively, the fiber gratings can bewritten directly into the photosensitive gain fiber, with an index anddopant profile that suppresses coupling to cladding modes in the fibergrating.

To sustain large spectral bandwidth, fiber grating 2404 has preferably aspectral bandwidth >20 nm. A splice 2407 (or an equivalent bulk opticlens assembly) is used to connect the output of fiber laser 2401 tononlinear fiber 2408 to be used for additional spectral broadening ofthe output of the fiber laser. For example fiber 2408 can comprise ahighly nonlinear dispersion-flattened holey fiber. In conjunction withsuch fiber, smooth broad-bandwidth spectral profiles with bandwidthsexceeding 100 nm can be generated. These spectral outputs can be useddirectly in high precision optical coherence tomography.

The pulses at the output of fiber 2408 are generally chirped and adispersion compensation module 2409 can be inserted after the outputfrom fiber 2408 for additional pulse compression. The dispersioncompensation module can be spliced directly to fiber end 2408 whenoptical fiber is used for dispersion compensation. Alternatively, thedispersion compensation module can comprise two (or one) bulk grating(or prism) pair(s). Such bulk optic elements for dispersion compensationare well known in the state of the art and are not further discussedhere. Coupling into and out of a bulk dispersion compensating module isobtained via lenses 2410 and 2411. The pulses generated after pulsecompression can be as short as 20-200 fs.

An additional highly nonlinear fiber 2412 (or a number of splicedtogether highly nonlinear fibers) is then used for the generation of thecoherent spectral continuum. These spectral continua can be subsequentlyused in precision frequency metrology.

Note that the discussion with respect to FIG. 27 serves only as anexample of the use of bi- or multi-temporal saturable absorbers in thegeneration of mass producible ultra-broad band, low noise spectralsources. Other modifications are obvious to anyone skilled in the art.These modifications can comprise for example the insertion of additionalfiber amplifiers after the output of fiber laser 2401 and theconstruction of an integrated all-fiber assembly substituting elements2408, 2409-2411 and 2412.

Though the discussion of the laser system with respect to FIG. 27 wasbased on the use of polarization maintaining fiber, non-polarizationmaintaining fiber can also be used to produce pulses with bandwidthcomparable to the bandwidth of the gain medium. In this case, saturableabsorbers with depth controlled ion implantation are also of greatvalue. Essentially, any of the prior art modelocked fiber laser systemsdescribed above (that were using saturable absorbers) can be improvedwith engineered bi- and multi-temporal saturable absorbers.Specifically, any of the cavity designs described in '427 and '848 toFermann et al. can be used for the generation of ultra-broadband opticalpulses in conjunction with bi- or multi-temporal saturable absorbers andwide-bandwidth fiber Bragg gratings.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention, as defined by the appended claims and equivalentsthereof.

1. A pulsed fiber laser amplification system comprising: a passivelymodelocked fiber oscillator outputting optical pulses, said passivelymodelocked fiber oscillator comprising a saturable absorber disposed ina temperature-controlled saturable absorber module and an oscillatorpump that optically pumps said passively modelocked fiber oscillator,wherein said saturable absorber exhibits a multi-temporal lifetime andis configured in such a way that Q-switched mode-locking evolves at alow oscillator pump power followed by cw mode locking at a higheroperational pump power that exceeds a threshold pump power for operationof said modelocked fiber oscillator; a fiber Bragg grating configured tocontrol intra-cavity dispersion of said passively modelocked fiberoscillator and configured as a mirror of said passively modelocked fiberoscillator; a fiber amplifier optically connected to said modelockedfiber oscillator via a first optical path, said amplifier amplifyingsaid optical pulses; an optical pump source optically connected to saidfiber amplifier; a pulse compressor optically coupled to receive saidamplified optical pulses output from fiber amplifier via a secondoptical path; and (i) a first optical tap in said first optical pathbetween said modelocked fiber oscillator and said fiber amplifier, and afirst feedback loop from said first tap to control said modelocked fiberoscillator based on measurement of output from said first optical tap,said first feedback loop from said first tap controlling at least saidoscillator pump, and (ii) a second optical tap in said second opticalpath between said fiber amplifier and said compressor, and a secondfeedback loop from said second tap to control said fiber amplifier basedon measurement of output from said second optical tap.
 2. The pulsedfiber laser amplification system of claim 1, further comprising avariable optical attenuator in the first optical path between saidmodelocked fiber oscillator and said fiber amplifier, said variableoptical attenuator having an adjustable transmission such that opticalenergy coupled from the modelocked fiber oscillator to the fiberamplifier is reduced.
 3. The pulsed fiber laser amplification system ofclaim 2, wherein said variable optical attenuator comprises apolarization selective optical component and provides an adjustabletransmission such that an amplitude of said optical pulses that arecoupled from said mode-locked fiber oscillator to said amplifier can bereduced.
 4. The pulsed fiber laser amplification system of claim 1,further comprising a first isolator in said first optical path from saidoscillator to said fiber amplifier, said first isolator comprising saidfirst tap.
 5. The pulsed fiber laser amplification system of claim 1,further comprising a second isolator in said second optical path fromsaid fiber amplifier to said compressor, said second isolator containingsaid second tap.
 6. The pulsed fiber laser amplification system of claim1, further comprising a temperature controller in thermal contact withsaid modelocked fiber oscillator to adjust operation of said modelockedfiber oscillator, said first feedback loop from said first tapcontrolling said temperature controller.
 7. The pulsed fiber laseramplification system of claim 1, further comprising a temperaturecontroller in thermal contact with said fiber amplifier to adjustoperation of said fiber amplifier, said second feedback loop from saidsecond tap controlling said temperature controller.
 8. The pulsed fiberlaser amplification system of claim 1, wherein said second feedback loopfrom said second tap controls said optical pump source.
 9. The pulsedfiber laser amplification system of claim 1, wherein said saturableabsorber module comprises a polarizing optical component.
 10. The pulsedfiber laser amplification system of claim 9, wherein said polarizingoptical component comprises at least one of a polarization beamsplitter,a variable waveplate, a polarization maintaining fiber, and a rotatableoptical fiber.
 11. The pulsed fiber laser amplification system of claim9, wherein a polarization extinction ratio of the saturable absorbermodule is above about 23 dB.
 12. The pulsed fiber laser amplificationsystem of claim 1, wherein said modelocked fiber oscillator comprises acavity fiber assembly module optically coupled to said saturableabsorber module.
 13. The pulsed fiber laser amplification system ofclaim 12, wherein said cavity fiber assembly module is optically coupledto said saturable absorber module by an optical fiber pigtail.
 14. Thepulsed fiber laser amplification system of claim 13, wherein themodelocked fiber oscillator comprises a resonant cavity, and a length ofsaid optical fiber pigtail can be varied to contribute to a desiredgroup delay dispersion of the cavity.
 15. The pulsed fiber laseramplification system of claim 12, wherein said cavity fiber assemblymodule is coupled to a temperature control component configured toadjust the temperature of a gain fiber in the cavity fiber assemblymodule to stabilize gain and reduce noise.
 16. The pulsed fiber laseramplification system of claim 1, wherein said saturable absorberexhibits a multi-temporal lifetime with a response time of less thanabout 20 ps and a recovery time of greater than about 50 ps.
 17. Thepulsed fiber laser amplification system of claim 16, wherein saidrecovery time is greater than about 100 ps.
 18. The pulsed fiber laseramplification system of claim 16, wherein said response time is lessthan about 5 ps.
 19. The pulsed fiber laser amplification system ofclaim 1, wherein said saturable absorber comprises multiple quantumwells or bulk semiconductor films, and wherein said saturable absorberis configured with non-uniform ion-implantation which, in combinationwith one or more implantation parameters, achieves said multi-temporallifetime in the presence of optical excitation with short opticalpulses.
 20. The pulsed fiber laser amplification system of claim 1,wherein said passively modelocked fiber oscillator comprises a Yb-dopedgain fiber.
 21. The pulsed fiber laser amplification system of claim 1,wherein said passively modelocked fiber oscillator comprises Yb-dopedpolarization maintaining fiber.
 22. The pulsed fiber laser amplificationsystem of claim 1, wherein said fiber Bragg grating is configured as anoutput mirror of said passively modelocked fiber oscillator.
 23. Thepulsed fiber laser amplification system of claim 1, wherein said fiberBragg grating is arranged such that the total intra-cavity dispersion isnegative.
 24. The pulsed fiber laser amplification system of claim 1,wherein said passively modelocked fiber oscillator comprises Yb-dopedpolarization maintaining fiber.
 25. The pulsed fiber laser amplificationsystem of claim 1, wherein said saturable absorber comprises: a film ofa semiconductor material implanted with high energy ions at apenetration depth which differs from the penetration depth of opticalsignals reflected from said saturable absorber, said semiconductormaterial having an ion implantation depth profile selected to providesaturable absorption having a multi-temporal carrier relaxation.
 26. Thepulsed fiber laser amplification system of claim 1, further comprising:a pulse stretcher disposed upstream from said fiber amplifier, whereinsaid fiber amplifier is configured such that attenuating an amplitude ofthe optical pulses coupled from said mode-locked fiber oscillator tosaid amplifier reduces a pulse width at an output of said compressor.27. The pulsed fiber laser amplification system of claim 1, furthercomprising: a spectral filter disposed external to the modelocked fiberoscillator and between said oscillator and said amplifier, said spectralfilter configured to receive said optical output of said modelockedfiber oscillator prior to reaching said amplifier, said spectral filterhaving a spectral transmission with a band edge that overlaps saidspectral power distribution of said optical output of said modelockedfiber oscillator to attenuate a portion of said spectral powerdistribution and thereby reduce the spectral bandwidth, the pulse widthof said optical pulses coupled from said modelocked fiber oscillator tosaid amplifier thereby being reduced, wherein said spectral filterreduces the spectral bandwidth to less than about 12 nanometers.
 28. Thepulsed fiber amplification system of claim 27, wherein said spectralfilter comprises a fiber Bragg grating.
 29. The pulsed fiber laseramplification system of claim of claim 1, wherein compressed high-powershort laser pulses at an output of said pulse compressor comprise anoptical power of at least about 200 mW and a pulse duration of about 200femtoseconds or less.
 30. The pulsed fiber laser amplification system ofclaim 1, further comprising a pre-compressor in the first optical pathbetween said modelocked fiber oscillator and said fiber amplifier. 31.The pulsed fiber laser amplification system of claim 30, wherein saidpre-compressor comprises a spectral filter or a dispersive opticalelement. 32-37. (canceled)
 38. The pulsed fiber amplification system ofclaim 27, wherein said spectral filter has a spectral bandwidth ofbetween about 5 and 12 nm.